Added mutex-functions to pthreads.c

Following functions were added:
- pthread_mutex_init
- pthread_mutex_trylock
- pthread_mutex_lock
- pthread_mutex_unlock
Added mutex-functions to pthreads.c

Following functions were added:
- pthread_mutex_init
- pthread_mutex_trylock
- pthread_mutex_lock
- pthread_mutex_unlock
Merge branch 'sidebranch' of https://github.com/Morrow3/zeke into sidebranch
Added headers and mutex typedefinitions to threads.h
Fixed missing enum from ptrhead.h
Integrated POSIX mutex code