Bring back nanosleep from the cold.

The addition of the nanosleep syscall was correctly added to
libc/sys/Makefile so that it is renamed as _thread_sys_nanosleep().
This syscall is one of those that libc_r has to re-implement because
the only behaviour is to block the process. So libc_r just ignores the
fact that a nanosleep syscall exists and goes its own way - as it has
done all along .... and now it does again. And now a simple program
can sleep again. Phew.

lib/libc_r/uthread/uthread_nanosleep.c

@@ -37,7 +37,7 @@
 #include "pthread_private.h"
 
 int
-_thread_nanosleep(const struct timespec * time_to_sleep,
+nanosleep(const struct timespec * time_to_sleep,
 		  struct timespec * time_remaining)
 {
 	int             ret = 0;

lib/libkse/thread/thr_nanosleep.c

@@ -37,7 +37,7 @@
 #include "pthread_private.h"
 
 int
-_thread_nanosleep(const struct timespec * time_to_sleep,
+nanosleep(const struct timespec * time_to_sleep,
 		  struct timespec * time_remaining)
 {
 	int             ret = 0;

lib/libpthread/thread/thr_nanosleep.c

@@ -37,7 +37,7 @@
 #include "pthread_private.h"
 
 int
-_thread_nanosleep(const struct timespec * time_to_sleep,
+nanosleep(const struct timespec * time_to_sleep,
 		  struct timespec * time_remaining)
 {
 	int             ret = 0;