projtool.pl: add new listalltags subcommand

projtool.pl: add new listalltags subcommand

There is a preexisting toolbox/show-tags.pl utility.

While it definitely works for a list of all tags and their
counts, that's all it can do -- it cannot show which projects
the tags belong to.

Add a new listalltags subcommand to projtool.pl that brings
some additional flexibility.  It can list the tags only
without their counts as well as include the projects that
contain them.  In addition, it can filter tag names by a
Perl regular expression.

As with the show-tags.pl utility, the new listalltags subcommand
makes use of the gitweb.index.cache file in order to provide
the results very quickly.

In addition, there is a --cache-info option that can show the
current state of the gitweb.index.cache file including how
out-of-date it might be.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>