relibc: implement pthread mutex prioceiling get/replace

Added prioceiling: c_int field to RlctMutex struct and real
implementations for prioceiling() and replace_prioceiling()
methods. Previously both returned Ok(0) silently (worse than
ENOSYS - caller believes operation succeeded).

pthread_mutex_t size updated from 12 to 16 bytes to accommodate
the new field while maintaining ABI alignment via #[repr(C)]
union with c_int alignment.

Cross-referenced with POSIX pthread_mutex_getprioceiling(3) and
Linux glibc nptl pthread_mutex_getprioceiling.c.

The prioceiling is stored in the mutex struct, set at construction
from pthread_mutexattr_getprioceiling. getprioceiling returns
the stored value; replace_prioceiling updates it and returns the
old value.

src/header/bits_pthread/mod.rs

@@ -41,7 +41,7 @@ pub union pthread_barrierattr_t {
 /// The `pthread_mutex_t` type provided in [`sys/types.h`](crate::header::sys_types).
 #[repr(C)]
 pub union pthread_mutex_t {
-    __relibc_internal_size: [c_uchar; 12],
+    __relibc_internal_size: [c_uchar; 16],
     __relibc_internal_align: c_int,
 }
 /// The `pthread_mutexattr_t` type provided in [`sys/types.h`](crate::header::sys_types).