ipfw: treat ipv6 address with zero mask as 'any'

Make the behaviour similar for both IPv4 and IPv6. Also add
the corresponding tests.

PR: 294733
Differential Revision: https://reviews.freebsd.org/D56618

(cherry picked from commit 2872268c7f6d473aae9b02ebb5d2c24fc2cff9b1)