refactor thrd_sleep and nanosleep in terms of clock_nanosleep

for namespace-safety with thrd_sleep, this requires an alias, which is
also added. this eliminates all but one direct call point for
nanosleep syscalls, and arranges that 64-bit time_t conversion logic
will only need to exist in one file rather than three.

as a bonus, clock_nanosleep with CLOCK_REALTIME and empty flags is now
implemented as SYS_nanosleep, thereby working on older kernels that
may lack POSIX clocks functionality.

src/thread/thrd_sleep.c

@@ -1,10 +1,11 @@
 #include <threads.h>
+#include <time.h>
 #include <errno.h>
 #include "syscall.h"
 
 int thrd_sleep(const struct timespec *req, struct timespec *rem)
 {
-	int ret = __syscall(SYS_nanosleep, req, rem);
+	int ret = -__clock_nanosleep(CLOCK_REALTIME, 0, req, rem);
 	switch (ret) {
 	case 0:      return 0;
 	case -EINTR: return -1; /* value specified by C11 */