pg_upgrade to clusters with a different WAL segment size

Hi hackers,

The thread for the recent change to allow setting the WAL segment size at
initdb time [0] included a bit of discussion regarding pg_upgrade [1],
where it was suggested that relaxing an error check (presumably in
check_control_data()) might be enough to upgrade servers to a different WAL
segment size.

We've had success with our initial testing of upgrades to larger WAL
segment sizes, including post-upgrade pgbench runs.  Beyond adjusting
check_control_data(), it looks like the 'pg_resetwal -l' call in
copy_xact_xlog_xid() may need to be adjusted to ensure that the WAL
starting address is set to a valid value.

Allowing changes to the WAL segment size during pg_upgrade seems like a
nice way to avoid needing a dump and load, so I would like to propose
adding support for this.  I'd be happy to submit patches for this in the
next commitfest.

Nathan

[0] https://git.postgresql.org/gitweb/?p=postgresql.git;a=commit;h=fc49e24fa69a15efacd5b8958115ed9c43c48f9a
[1] https://www.postgresql.org/message-id/20160826043926.pwkz45ksxpmfn4g6%40alap3.anarazel.de