bad termination of responses to 7-bit OSC colour queries

[Stuart Henderson]

If rxvt-unicode receives a colour query via the 7-bit control sequence,
it just uses a single ESC rather than the correct terminator (BEL,
ST, or ESC \):

$ printf '\033]10;?\033\\'; hexdump -C 
^[]10;rgb:cccc/cccc/cccc^[
00000000  1b 5d 31 30 3b 72 67 62  3a 63 63 63 63 2f 63 63  |.]10;rgb:cccc/cc|
00000010  63 63 2f 63 63 63 63 1b  0a                       |cc/cccc..|
00000019

Essentially rxvt_term::process_osc_seq and the api passing a char around
only handles the single char (BEL/ST) and not the 7-bit sequences.

The combination of the fix in 9.31 (where rxvt-unicode starts to respond
to additional OSC queries) with a recent change in tmux causing it to
make OSC 10 and OSC 11 queries) results in junk being entered when
attaching to recent tmux from 9.31.

(The tmux commit introducing this is
https://github.com/tmux/tmux/commit/a41a92744188ec5c8a8d4ddc100ec15b52d04603
and a follow-up recognises BEL as well as ST
https://github.com/tmux/tmux/commit/c0031f8b8581f7fc2d75cabade596be68f85aa81)

As a temporary workaround I've patched rxvt_term::process_color_seq
to override resp from the caller and just use BEL; this has the desired
effect but obviously overrides what rxvt-unicode is trying to do with
passing the terminator up via resp.

It looks like a correct fix maintaining what rxvt-unicode is trying to
do would involve a larger change to use a string instead of char for resp.