Fix regression: functions containing domain (ip ip6)

Fix regression: functions containing domain (ip ip6)

Commit 729e3e introduced support for individual evaluation of rules
per-domain. As part of that, inside a domain statement, execution forks
into two inner rules and then individually executed.

However, the inner rule is entered starting with level $lev, which can
never be zeroed back, since its contents are collected via a call to
collect_tokens(), which is a zero-summed collection.

This worked on most cases, since domain is usually defined on level
zero. A notable exception is when domain (ip ip6) is defined inside a
function.

Fix this by entering the inner rule with an initial zero level and add a
test case to the test suite. This is a fix for a ferm 2.1 regression.