subrepo: use per-type config options to enable subrepos
We change subrepos.allowed from a list of allowed subrepo types to
a combination of a master switch and per-type boolean flag.
If the master switch is set, subrepos can be disabled wholesale.
If subrepos are globally enabled, then per-type options are
consulted. Mercurial repos are enabled by default. Everything else
is disabled by default.