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/string/strncpy.c

@@ -2,7 +2,7 @@
 
 char *__stpncpy(char *, const char *, size_t);
 
-char *strncpy(char *d, const char *s, size_t n)
+char *strncpy(char *restrict d, const char *restrict s, size_t n)
 {
 	__stpncpy(d, s, n);
 	return d;