x11/rxvt-unicode: Update to 9.31_2 (tmux fix)

[hsw_bitmark.com]

Summary

prevent tmux (3.2a) from getting the colour info into the input buffer
which shows as if the text below was typed:

10;rgb:ffff/ffff/ffff]11;rgb:0000/0000/8b8b

the original patch from OpenBSD is here:

https://cvsweb.openbsd.org/cgi-bin/cvsweb/~checkout~/ports/x11/rxvt-unicode/patches/patch-src_command_C?rev=1.6&content-type=text/plain

the OpenBSD patch has this description:

OSC commands returning the colour sequence must be terminated by either
of ST, ESC \, or BEL. rxvt-unicode tries to use the same type of termination
as was in the query but doesn't correctly handle the multiple-char sequence
used for 7-bit queries. Force to using ESC \ instead for now.

Signed-off-by: Christopher Hall <[e-mail]>