Add Set/GetThreadPriority

Add Set/GetThreadPriority using set/getpriority rather than pthread_set/getschedparam as suggested in other patches (https://gist.github.com/justincc/31e52218d098529b4696 or https://github.com/Hzj-jie/mono/commit/4c7bfed18080119a65616181b24542af6d8e3eef)
This way of changing the priority has the benefit of not affecting other processes running on the system (SCHED_FIFO and SCHED_RR automatically has a higher priority than SCHED_NORMAL processes even with the lowest priority: 1).
It requires root permissions to set priorities higher than normal, but this is also the case on windows with the Microsoft implemented .NET framework.
The patch was originally made for the 3.2.8 version and ported to HEAD.
The original patch can be found here: https://gist.github.com/aasimon/c8ae6fc3cf5d9b82b6ca
Mono uses C89.