Re: POC: make mxidoff 64 bits

Here's yet another patch version. I spent the day reviewing this in
detail and doing little cleanups here and there. I squashed the commits
and wrote a proper commit message.

One noteworthy refactoring is in pg_upgrade.c, to make it more clear (to
me at least) how upgrade from version 9.2 and below now works. It was
actually broken when I tested it. Not sure if I had broken it earlier or
if it never worked, but in any case it works now.

I also tested upgrading a cluster from an old minor version, < 9.3.5,
where the control file has a bogus oldestMultiXid==1 value (see commit
b6a3444fa6). As expected, you get a "could not open file" error:

> Performing Upgrade
> ------------------
> Setting locale and encoding for new cluster ok
> ...
> Deleting files from new pg_multixact/members ok
> Deleting files from new pg_multixact/offsets ok
> Converting pg_multixact files
> could not open file "/home/heikki/pgsql.93stable/data/pg_multixact/offsets/0000": No such file or directory
> Failure, exiting

I don't think we need to support that case. I hope there are no clusters
in that state still in the wild, and you can work around it by upgrading
to 9.3.5 or above and letting autovacuum run. But I wonder if a
pre-upgrade check with a better error message would still be worthwhile.

Ashutosh, you were interested in reviewing this earlier. Would you have
a chance to review this now, before I commit it? Alexander, Alvaro,
would you have a chance to take a final look too, please?

- Heikki