fs-util: introduce fchmod_opath()

fchmod(2) still doesn't take file descriptors opened with O_PATH.

src/basic/fs-util.h

@@ -33,6 +33,7 @@ int readlink_and_make_absolute(const char *p, char **r);
 int chmod_and_chown(const char *path, mode_t mode, uid_t uid, gid_t gid);
 
 int fchmod_umask(int fd, mode_t mode);
+int fchmod_opath(int fd, mode_t m);
 
 int fd_warn_permissions(const char *path, int fd);