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/include/time.h

@@ -4,6 +4,7 @@
 #include "../../include/time.h"
 
 hidden int __clock_gettime(clockid_t, struct timespec *);
+hidden int __clock_nanosleep(clockid_t, int, const struct timespec *, struct timespec *);
 
 hidden char *__asctime_r(const struct tm *, char *);
 hidden struct tm *__gmtime_r(const time_t *restrict, struct tm *restrict);

src/thread/thrd_sleep.c

 #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 */

src/time/clock_nanosleep.c

@@ -2,8 +2,12 @@
 #include <errno.h>
 #include "syscall.h"
 
-int clock_nanosleep(clockid_t clk, int flags, const struct timespec *req, struct timespec *rem)
+int __clock_nanosleep(clockid_t clk, int flags, const struct timespec *req, struct timespec *rem)
 {
-	int r = -__syscall_cp(SYS_clock_nanosleep, clk, flags, req, rem);
-	return clk == CLOCK_THREAD_CPUTIME_ID ? EINVAL : r;
+	if (clk == CLOCK_THREAD_CPUTIME_ID) return EINVAL;
+	if (clk == CLOCK_REALTIME && !flags)
+		return -__syscall_cp(SYS_nanosleep, req, rem);
+	return -__syscall_cp(SYS_clock_nanosleep, clk, flags, req, rem);
 }
+
+weak_alias(__clock_nanosleep, clock_nanosleep);

src/time/nanosleep.c

@@ -3,5 +3,5 @@
 
 int nanosleep(const struct timespec *req, struct timespec *rem)
 {
-	return syscall_cp(SYS_nanosleep, req, rem);
+	return __syscall_ret(-__clock_nanosleep(CLOCK_REALTIME, 0, req, rem));
 }