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/stat/stat.c

@@ -2,7 +2,7 @@
 #include "syscall.h"
 #include "libc.h"
 
-int stat(const char *path, struct stat *buf)
+int stat(const char *restrict path, struct stat *restrict buf)
 {
 	return syscall(SYS_stat, path, buf);
 }