src: allow variables in the chain priority specification

src: allow variables in the chain priority specification

This patch allows you to use variables in chain priority definitions,
e.g.

define prio = filter
define prionum = 10
define prioffset = "filter - 150"

add table ip foo
add chain ip foo bar { type filter hook input priority $prio; }
add chain ip foo ber { type filter hook input priority $prionum; }
add chain ip foo bor { type filter hook input priority $prioffset; }

Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>