winpthreads: mutex.c: update `pthread_mutexattr_*` functions

Interpret `pthread_mutexattr_t` as a union consisting of bit fields.

This change makes it easier to understand how `pthread_mutexattr_*` functions
operate on `pthread_mutexattr_t` objects and provide support for new attributes.

Remove macro `PTHREAD_PRIO_MULT` from pthread.h; this constant is not defined
by POSIX and is only used by `pthread_mutexattr_{set,get}prioceiling` functions.

Signed-off-by: Kirill Makurin <maiddaisuki@outlook.com>
Signed-off-by: LIU Hao <lh_mouse@126.com>