src: default to numeric UID and GID listing

src: default to numeric UID and GID listing

Like iptables-save, print UID and GID as numeric values by default.

Add a new option `-u' to print the UID and GID names as defined by
/etc/passwd and /etc/group.

Note that -n is ignored after this patch, since default are numeric
printing for UID and GID.

Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>