util: promote rmrf() from tmpfiles.c, fix silent mksubsys() skip

rmrf() is needed outside tmpfiles.c.  The move also deduplicates the
nftw callback: the contents-only removal used by tmpfiles 'D' entries
is now rmcontents(), sharing the callback with rmrf().

mksubsys() did nothing at all when the user could not be resolved, no
directory and no message, and callers had no way to tell.  Now the
directory is always created, ownership is best effort, and an unknown
user is warned about.

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

src/util.h

@@ -72,6 +72,8 @@ int   getcuser     (char *buf, size_t len);
 int   getcgroup    (char *buf, size_t len);
 
 int   mksubsys     (const char *dir, mode_t mode, char *user, char *group);
+int   rmcontents   (const char *path);
+int   rmrf         (const char *path);
 
 char *fslurp       (size_t *lenp, const char *fmt, ...)    __attribute__ ((format (printf, 2, 3)));
 char *vfslurp      (size_t *lenp, const char *fmt, va_list ap);