add stub versions of some missing optional pthread interfaces

priority inheritance is not yet supported, and priority protection
probably will not be supported ever unless there's serious demand for
it (it's a fairly heavy-weight feature).

per-thread cpu clocks would be nice to have, but to my knowledge linux
is still not capable of supporting them. glibc fakes them by using the
_process_ cpu-time clock and subtracting the thread creation time,
which gives seriously incorrect semantics (worse than not supporting
the feature at all), so until there's a way to do it right, it will
remain as a stub that always fails.

include/pthread.h

@@ -105,6 +105,9 @@ int pthread_mutex_timedlock(pthread_mutex_t *__restrict, const struct timespec *
 int pthread_mutex_destroy(pthread_mutex_t *);
 int pthread_mutex_consistent(pthread_mutex_t *);
 
+int pthread_mutex_getprioceiling(const pthread_mutex_t *__restrict, int *__restrict);
+int pthread_mutex_setprioceiling(pthread_mutex_t *__restrict, int, int *__restrict);
+
 int pthread_cond_init(pthread_cond_t *__restrict, const pthread_condattr_t *__restrict);
 int pthread_cond_destroy(pthread_cond_t *);
 int pthread_cond_wait(pthread_cond_t *__restrict, pthread_mutex_t *__restrict);
@@ -192,6 +195,8 @@ int pthread_atfork(void (*)(void), void (*)(void), void (*)(void));
 int pthread_getconcurrency(void);
 int pthread_setconcurrency(int);
 
+int pthread_getcpuclockid(pthread_t, clockid_t *);
+
 struct __ptcb {
 	void (*__f)(void *);
 	void *__x;

src/thread/pthread_attr_get.c

@@ -68,6 +68,11 @@ int pthread_condattr_getpshared(const pthread_condattr_t *restrict a, int *restr
 	return 0;
 }
 
+int pthread_mutexattr_getprotocol(const pthread_mutexattr_t *restrict a, int *restrict protocol)
+{
+	*protocol = PTHREAD_PRIO_NONE;
+	return 0;
+}
 int pthread_mutexattr_getpshared(const pthread_mutexattr_t *restrict a, int *restrict pshared)
 {
 	*pshared = *a>>31;

src/thread/pthread_getcpuclockid.c

+#include "pthread_impl.h"
+
+int pthread_getcpuclockid(pthread_t t, clockid_t *clockid)
+{
+	return ENOSYS;
+}

src/thread/pthread_mutex_getprioceiling.c

+#include "pthread_impl.h"
+
+int pthread_mutex_getprioceiling(const pthread_mutex_t *restrict m, int *restrict ceiling)
+{
+	return EINVAL;
+}

src/thread/pthread_mutex_setprioceiling.c

+#include "pthread_impl.h"
+
+int pthread_mutex_setprioceiling(pthread_mutex_t *restrict m, int ceiling, int *restrict old)
+{
+	return EINVAL;
+}

src/thread/pthread_mutexattr_setprotocol.c

+#include "pthread_impl.h"
+
+int pthread_mutexattr_setprotocol(pthread_mutexattr_t *a, int protocol)
+{
+	if (protocol) return ENOTSUP;
+	return 0;
+}