extensions: move extensions listing functions from mercurial.help
Includes some small fix-ups to comments in enabled() and disabled().
% Without -v
access log created - .hg/hgrc respected
% errors
% With -v
listening at http://localhost/ (bound to 127.0.0.1)
% errors
% With --prefix foo
listening at http://localhost/foo/ (bound to 127.0.0.1)
% errors
% With --prefix /foo
listening at http://localhost/foo/ (bound to 127.0.0.1)
% errors
% With --prefix foo/
listening at http://localhost/foo/ (bound to 127.0.0.1)
% errors
% With --prefix /foo/
listening at http://localhost/foo/ (bound to 127.0.0.1)
% errors