src: remove STMT_NAT_F_INTERVAL flags and interval keyword

src: remove STMT_NAT_F_INTERVAL flags and interval keyword

STMT_NAT_F_INTERVAL is not useful, the keyword interval can be removed
to simplify the syntax, e.g.

 snat to ip saddr map { 10.141.11.4 : 192.168.2.2-192.168.2.4 }

This patch reworks 9599d9d25a6b ("src: NAT support for intervals in
maps").

Do not remove STMT_NAT_F_INTERVAL yet since this flag is needed for
interval concatenations coming in a follow up patch.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>