Fix IPv6 range parser

As reported on the shorewall-users mailing list, using an IPv6 range in /etc/shorewall6/masq of the form [2001:470:a:227::2]-[2001:470:a:227::10] (which is the format documented in the shorewall6-masq(5) manual page) produces the message ERROR: Correct address range syntax is '[<addr1>-<addr2>]'.

This is a bug in the IPv6 range parser. The range parser should be fixed to read addresses specified in the documented form. Additionally, to protect against regressions in already working configurations, the IPv6 range parser should continue accept address ranges in the format indicated by the current error message.