threads: remove WINAPI, implement priority for pthreads

threads: remove WINAPI, implement priority for pthreads

Also:
[fix] threads: fix race condition when stopping thread - cherry pick merge

Also, from jimfcarroll:
[fixed] AirPlayServer now complies with the new CThread constructor requirements (that is, you must supply a thread name).
replace custom thread local storage with the XbmcThreads::ThreadLocal template.
[cosmetic] removed incorrect hungarian notation.
[cleanup] Removed XSyncUtils. Moved the GlobalMemoryStatus call to XMemUtil. Cleaned up code that used the #define INFINITE from XSyncUtils.h
[cleanup] Some miscellaneous cleanup. Fixed initialization warning in LCD. Removed some unused functions and members.
[cleanup] PAPlayer warning due to constructor initialization list order.