make setproctitle()'s /proc/pid/stat parsing safe

make setproctitle()'s /proc/pid/stat parsing safe

it turns out that our parsing of /proc/pid/stat was not safe in general
(though probably safe for lxc, since our executable names do not contain
spaces).

Let's fix this by looking backwards through the file for ), and then
continuing on from there.

This was reported to me by Solar Designer, who pointed me to this thread:
https://twitter.com/solardiz/status/1634204168545001473

Indeed, this is a lot of tap dancing to work around the kernel's 16
character executable limit. Perhaps I'll send a kernel patch to raise that
limit next.

Signed-off-by: Tycho Andersen <tycho@tycho.pizza>