import via "anonymous" ssh (BB-7620)

Various projects have to move from opensolaris.org over the next month or so, but access to hg.opensolaris.org is only allowed via ssh for security reasons.

There's a special "anon" (anonymous) account for hg.opensolaris.org that allows ssh pull access and does not require a password or ssh key.

It would be handy if the import code feature allowed this setup. Here's an example of the public clone steps normally taken for an opensolaris.org repository:

hg clone ssh://anon@hg.opensolaris.org/hg/pkg/gate