subprocess: implement proper detached processes on POSIX

The previous method for this sucked: for every launched detached
process, it started a thread, which then would leak if the launched
process didn't end before the player uninitialized. This was very racy
(although I bet the race condition wouldn't trigger in a 100 years), and
wasteful (threads aren't a cheap resource).

Implement it for POSIX directly. posix_spawn() has no direct support for
this, so we need to do it ourselves with fork(). We could probably do it
without fork(), and attempt to collect the PID in another thread. But
then we'd either have a waiting thread again, or we'd need to do an
unsafe waitpid(-1, ...) call. (POSIX process management sucks so badly,
how did they even manage this. Hopefully I'm just missing something, but
I'm not.) So now we depend on both posix_spawn() _and_ fork(), isn't it
fun?

Also call setsid(), to essentially detach the child process from the
terminal. (Otherwise it can receive various signals from the terminal,
which is probably not what you want.) posix_spawn() adds
POSIX_SPAWN_SETSID in newer POSIX releases, but we don't want to rely on
this yet.

The posix_spawnp() call is duplicated, but this is better than somehow
trying to unify the code paths.

Only somewhat tested, so enjoy the bugs.

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