add -r flag to make mountd do reverse resolve of ipaddress on the fly

For those that want "traditional" showmount -a behavior from their mountd
(hostname:/path instead of ipaddr:/path). This patch adds a '-r' flag that
does a reverse-resolve for each IP address listed in the rmtab when
a dump operation is called.

Probably not a good idea for those concerned about performance, but since it's
not the default option, I don't see it being an issue.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Neil Brown <neilb@suse.de>