Fix GLIBC/Linux-only codepath in tracker_sched_idle()

Previously we were using the GLIBC sched_setscheduler() function. This is Linux-only and also doesn't work as documented -- it claims to set policy for the whole process, but in fact only affects the calling thread.

We now use the pthread_setschedparam() function from pthreads, which also only affects the calling thread, and is also portable beyond Linux and GLIBC.

Fixes tracker#170 (closed)