Fixup incorrect expansion in pfctl.

MFC pfctl fix (parse.y 1.641, pfctl_parser.c 1.300, pfctl_parser.h 1.104)

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Fixup incorrect expansion of the networking mask for dynamic interface
specifications under certain circumstances resulting in potentially
elevated access permissions for IPv6 traffic.  Reported by sthen@;
ok henning benno sthen
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Imported to Bitrig by patrick@.