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>

etc/login.defs

@@ -229,7 +229,7 @@ SYS_UID_MAX		  999
 # Extra per user uids
 SUB_UID_MIN		   100000
 SUB_UID_MAX		600100000
-SUB_UID_COUNT		    10000
+SUB_UID_COUNT		    65536
 
 #
 # Min/max values for automatic gid selection in groupadd(8)
@@ -242,7 +242,7 @@ SYS_GID_MAX		  999
 # Extra per user group ids
 SUB_GID_MIN		   100000
 SUB_GID_MAX		600100000
-SUB_GID_COUNT		    10000
+SUB_GID_COUNT		    65536
 
 #
 # Max number of login(1) retries if password is bad