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/process/posix_spawnattr_getsigmask.c

@@ -1,6 +1,6 @@
 #include <spawn.h>
 
-int posix_spawnattr_getsigmask(const posix_spawnattr_t *attr, sigset_t *mask)
+int posix_spawnattr_getsigmask(const posix_spawnattr_t *restrict attr, sigset_t *restrict mask)
 {
 	*mask = attr->__mask;
 	return 0;