tmpfiles: set mode and owner of d/D directories through an fd

makedir() swallows EEXIST, so the chmod branch for directories that
already existed never ran, while chown() ran every time.  On finix
/var/empty is immutable, which gave a warning on every boot even
though the owner was already correct.

mksubsys() is the original of this code and has the same shape, the
dbus plugin uses it for /tmp/dbus.  Both now go through dirperm(),
which opens the directory and uses fstat/fchmod/fchown on the fd,
changing only what differs.  A failed chown in mksubsys() is a warning
now rather than err(1), PID 1 should not exit over a directory it
cannot adjust.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/util.h

@@ -68,6 +68,7 @@ int   getgroup     (const char *group);
 int   getcuser     (char *buf, size_t len);
 int   getcgroup    (char *buf, size_t len);
 
+int   dirperm      (const char *path, mode_t mode, uid_t uid, gid_t gid);
 int   mksubsys     (const char *dir, mode_t mode, char *user, char *group);
 
 int   fnread       (char *buf, size_t len, char *fmt, ...) __attribute__ ((format (printf, 3, 4)));