Signed-off-by: Rosen Penev <rosenp@gmail.com>

Signed-off-by: Rosen Penev <rosenp@gmail.com>

On linux-rt, kernel IRQ threads are configured with priority=50, and
this change configures MPD somewhat below that priority, leaving some
room for other programs to be configured in between.

Closes https://github.com/MusicPlayerDaemon/MPD/issues/643

There is a POSIX definition for sched_setscheduler(), but Linux does
not implement that; instead of changing the process's scheduler, it
only affects one thread.  This has caused some confusion among
application developers and C library developers.

While glibc implements Linux semantics, Musl has made their
sched_setscheduler() function an always-failing no-op, causing the
error message "sched_setscheduler failed: Function not implemented".

 http://git.musl-libc.org/cgit/musl/commit/src/sched/sched_setscheduler.c?id=1e21e78bf7a5c24c217446d8760be7b7188711c2

Instead of relying on the C library which may be unreliable here, we
now roll our own system call wrapper.

Closes #218

Juse in cas glibc gets a wrapper for the system call which would then
conflict with ours.

Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Reduce header clutter.

This only applies to linux systems.  Here, sched_setscheduler() is
called to get realtime scheduling.  With this patch, the return value
of this function is now checked and a warning / error message is
generated if it fails.

Bionic doesn't have the SYS_* macros.

Use it if it exists, but don't insist if the C library is poor
(e.g. Bionic/Android).

The update thread should not affect the rest of the system, therefore
set "idle" priority, and let it only run when nobody else is using the
resources.