Tweak uid/gid map default configuration

- Use an allocation of 65536 uids and gids to allow for POSIX-compliant
   user owned namespaces.
 - Don't allocate a uid/gid map to system users.
   Unfortunately checking for --system isn't quite enough as some
   distribution wrappers always call useradd without --system and take care
   of choosing a uid and gid themselves, so also check whether the
   requested uid/gid is in the user range.

This is taken from a patch I wrote for Ubuntu a couple years ago and
which somehow didn't make it upstream.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>

libmisc/find_new_sub_uids.c

@@ -58,7 +58,7 @@ int find_new_sub_uids (const char *owner,
 
 	min = getdef_ulong ("SUB_UID_MIN", 100000UL);
 	max = getdef_ulong ("SUB_UID_MAX", 600100000UL);
-	count = getdef_ulong ("SUB_UID_COUNT", 10000);
+	count = getdef_ulong ("SUB_UID_COUNT", 65536);
 
 	if (min > max || count >= max || (min + count - 1) > max) {
 		(void) fprintf (stderr,