use restrict everywhere it's required by c99 and/or posix 2008

to deal with the fact that the public headers may be used with pre-c99
compilers, __restrict is used in place of restrict, and defined
appropriately for any supported compiler. we also avoid the form
[restrict] since older versions of gcc rejected it due to a bug in the
original c99 standard, and instead use the form *restrict.

src/thread/pthread_sigmask.c

@@ -3,7 +3,7 @@
 #include <pthread.h>
 #include "syscall.h"
 
-int pthread_sigmask(int how, const sigset_t *set, sigset_t *old)
+int pthread_sigmask(int how, const sigset_t *restrict set, sigset_t *restrict old)
 {
 	int ret;
 	if ((unsigned)how - SIG_BLOCK > 2U) return EINVAL;