Add an unvis function and use it on file names in mtree specs.

This should correct the problem that ./bin/[ was missing from the
base.tgz set, despite being listed in src/distrib/sets/base/mi
and being present in METALOG.  The corresponding entry in
METALOG.sanitised has ./bin/\133 instead of ./bin/[, and that made
join.awk omit it.

XXX: The unvis() implementation in join.awk handles only a subset
of the syntax, but it's probably good enough for now.

XXX: The file names should probably be canonicalised by
vis(unvis(name)), but at present none of the file names in the set
lists really need it.

XXX: It may be a bug that entries in the set lists without
corresponding entries in METALOG are silently ignored by join.awk.