GMP repo serving

[Torbjorn Granlund]

In order to get repository web access under the https hat, I have moved
the URL (yet again).  The repositories are now at https://gmplib.org/repo/.

The web visible repos are rsynced from the live repositories to a
read-only location.  There is a delay of a few minutes now, this will be
fixed by letting a checkin hook trigger these rsyncs.

The old location gmplib.org:8000 works again after some weeks of
blackhole behaviour.  The intermediate hg.gmplib.org:8000 works, but
goes to the live repository for now, but that should change after a DNS
TTL times out.

I believe all of the gmplib.org contents can now be accessed over https.
Any http connection will be redirected to https.  Plain old ftp
downloads still work, but the same files are preferably accessed at
https://gmplib.org/download/.

Please let us know if something doesn't work.

Torbjörn