fs-util: add access_fd() which is like access() but for fds

Linux doesn't have faccess(), hence let's emulate it. Linux has access()
and faccessat() but neither allows checking the access rights of an fd
passed in directly.

src/basic/fs-util.h

@@ -98,3 +98,5 @@ static inline void unlink_and_free(char *p) {
         free(p);
 }
 DEFINE_TRIVIAL_CLEANUP_FUNC(char*, unlink_and_free);
+
+int access_fd(int fd, int mode);