sys: [Linux] remove/replace obsolete timing and sleep syscalls, slightly improve...

sys: [Linux] remove/replace obsolete timing and sleep syscalls, slightly improve timing

Uses CLOCK_MONOTONIC_RAW on Linux, this is unaffected by synchronisation
adjustments and is used by SDL_GetPerformanceCounter() (we'll use it
directly to support dedicated servers without SDL).

Removes gettimeofday() and usleep() which were marked obsolete and
removed in POSIX.1-2008 so the old comment // FIXME improve this check
was impossible to address. The gettimeofday() replacement is very well
tested now (sys_useclockgettime 1 is default even in div0-stable).

Removes host_maxwait as the problems it worked around were fixed.

Tidies up a little.

Signed-off-by: bones_was_here <bones_was_here@xonotic.au>