Add regexps capacity to PLIST_SUB.

This adds the possibility to use regular expressions for the makeplist
stage of the PLIST_SUB life.

From time to time, the values are too generic, and they get in the way of other
stuff.

This adds the possibility to have a VAR_regex=regex that will be used instead
of the VAR=string to search for possible replacements.

For example, in lang/perl5*, there is PERL_ARCH=mach, which will get
replaced in paths if a file is called, say "machine", will end up being
"%%PERL_ARCH%%ine". Adding PERL_ARCH_regex="\bmach\b" will ensure only
full words are replaced, so machine will stay machine, but "lib/mach/foo
"will still be replaced by "lib/%%PERL_ARCH%%/foo".

Reviewed by: bdrewery
Sponsored by: Absolight
Differential Revision: https://reviews.freebsd.org/D7335