perms: list available roles, plus several other changes...
(much thanks to Tony Finch!)
- show valid roles and their access rights, for the given repo ("-lr")
- check a role not just against the rc, but against the roles that this
specific repo actually uses, which may be a smaller subset. (This is the
most important improvement in this patch; the rest is just fluff!)
- fixup usage message
WARNING: minor backward compat breakage: 'ssh ... perms -l repo' no longer
works; the '-l' goes *after* the repo name now.