src: allow for variables in the log prefix string

src: allow for variables in the log prefix string

For example:

 define test = "state"
 define foo = "match"

 table x {
        chain y {
                ct state invalid log prefix "invalid $test $foo:"
        }
 }

This patch scans for variables in the log prefix string. The log prefix
expression is a list of constant and variable expression that are
converted into a constant expression from the evaluation phase.

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