Similar to xterm or urxvt holding shift before selecting text with the mouse
allows to override copying text. For example in tmux with "mode-mouse on" or
vim (compiled with --with-x), mc, htop, etc.

forceselmod in config.h sets the modifier to use this mode, by default
ShiftMask.

Signed-off-by: Hiltjo Posthuma <hiltjo@codemadness.org>

Otherwise, the rest of the input is interpreted as a new escape
sequence.
For the ESC character, ESC_START is re-set in tcontrolcode.

Control code are never displayed. It is not important if graphic
charset is displayed or not.

ATTR_GFX was used long time ago to detect when terminal was in
graphic mode. Today graphic mode is implemented using a charset
pointer, so ATTR_GFX is not needed anymore because graphic
condition can be detected directly checking if current charset
is GRAPHICS C0.

This patch fixes the bug introduced in
8f11e1cd03

To reproduce the bug:
1. Save cursor: printf '\e[s'
2. Load cursor: printf '\e[u'
3. Resize st window.
4. Load cursor again: printf '\e[u'

This misplaced break was causing an incorrect fall through
from DSR to DECSTBM.

Almost of the sequences execute their action in a separate function,
which is good because helps to read the full set of sequences
faster.

The patch 53105cf modified how control codes were detected, because
it tried to handle also C1 control codes (0x80-0x9f), that have
upper bit to 1, so they are multi byte character in utf8.
Code was checking the value of width in order to known that after
decoding the unicode point had a width of 1 byte, but it as incorrect
because this width is the columnb width.

Once a sequence is completed term.esc must return to 0, so
instead of repeating this expression in all the cases is
better put it at the end of the block.

From http://www.vt100.net/docs/vt510-rm/chapter4:

	*The VT510 ignores all following characters until it receives a
	 SUB, ST, or any other C1 control character.

So OSC, PM and APC sequence ends with a SUB (it cancels the sequence
and show a question mark as error), ST or any another C1 (8 bits)
code, or their C0 (7 bits) equivalent sequences (at this moment we
do not handle C1 codes, but we should). But it is also said that:

	Cancel  CAN
	1/8     Immediately cancels an escape sequence, control sequence,
		or device control string in progress. In this case, the
		VT510 does not display any error character.

	Escape  ESC
	1/11    Introduces an escape sequence. ESC also cancels any escape
		sequence, control sequence, or device control string in
		progress.

Currently tputc handles the case of too long control string waiting for
the end of control string.

Another case is when there is ESC character is encountered but is not
followed by '\\'.  In this case st stops processing control string,
but ESC character is ignored.

After this patch st processes ESC characters in control strings properly.

Test case:
printf '\e]0;abc\e[1mBOLD\e[0m'

Also ^[\ is actually processed in the code that handles ST.
According to ECMA-048 ST stands for STRING TERMINATOR and is used to
close control strings.

Works for both signed and unsigned char.

Currently the alternate screen get messed up on resize if it has
different colors or mode.

Before this patch executing
	printf '\e[10000000000I'
or
	printf '\e[10000000000Z'
resulted in long delay.

sel.alt is only changed by
	sel.alt = IS_SET(MODE_ALTSCREEN);

Thanks to Yuri Karaban for suggesting this!

These changes make -g correspond to <cols>x<rows> and honor it so non-tiling
window managers can work with the size hints afterwards. It also adds a -i
flag to force the window size. This is needed so -g keeps being useful in dwm.

The large and repeated expression used in memmove to indirect
the line can be simplified using a pointer, that makes more
clear where begins and where ends the movement.

Current CSI parsing code uses strtol to parse arguments and allows them
to be negative. Negative argument is not properly handled in tdeletechar
and tinsertblank and results in memory corruption in memmove.

Reproduce with printf '\e[-500@'

Patch also removes special handling for corner case and simplifies
the code.

Removed
	term.dirty[term.c.y] = 1
because tclearregion sets dirty flag.

tscrollup and tscrolldown do not use tsetdirt, but their code is
equivalent to

        tsetdirt(orig, term.bot-n);
        tsetdirt(orig+n, term.bot);

tclearregion also marks cleared lines as dirty.
In tscrolldown it sets lines from term.bot-n+1 to term.bot dirty, and in
tscrollup it sets lines from orig to orig+n-1 dirty.

In both functions all lines from orig to term.bot are effectively set
dirty, but in tscrolldown lines from orig+n to term.bot are set dirty
twice, and in tscrollup lines from orig to term.bot-n are set dirty
twice.

These patches make it clear which lines are set dirty and sets them
dirty once in each funciton.

techo compares signed char to '\x20'. Any character with code less then
'\x20' is treated as control character.  This way characters with MSB
set to 1 are considered control characters too.

Also this patch makes techo display DEL character as ^?.

To reprocuce the bug, enable echo mode using printf '\e[12l',
then type DEL character or any non-ASCII character.

Patch moves MODBIT to macros section and uses it in tselcs.

I found the SERRNO Macro slightly confusing, since you have to look
it up, if you don't know it already. A web search showed it does
not seem to be any kind of standard. Also there was no reason in
the commit log when it was introduced in 2009. As you can see it
also leads to new patches, which don't use this macro (probably the
author did not know about it).