[MERGE] [0.11] Working bzr+ssh://, make "bzr serve --inet" read-only by default.

[Andrew Bennetts]

Hi all,

http://people.ubuntu.com/~andrew/bzr/bzr-ssh-testing/ has some changes I'd like
to see merged, ideally for 0.11 despite the freeze:

  * make bzr+ssh:// URLs, added in previous smart server merges, work.  Most of
    the infrastructure was already there, but wasn't fully hooked up or tested.
    This branch rectifies that.  I'd really love for this to get in 0.11, as it
    will greatly increase the number of people that will experiment with the new
    smart server work.

  * adds a --allow-writes option "bzr serve" command (and default to only
    allowing read-only access).  This will make it harder for people to shoot
    themselves in the foot.  It would be particularly useful for serving
    anonymous bzr branches out of inetd -- so again, significantly boosting the
    utility of the smart server work so far.

So, I'm both seeking reviews of this code, and opinions on if it's ok to make an
exception for merging this new functionality into 0.11.

-Andrew.