Rename backup_label to recovery_control

Hackers,

I was recently discussing the complexities of dealing with pg_control
and backup_label with some hackers at PGConf NYC, when David Christensen
commented that backup_label was not a very good name since it gives the
impression of being informational and therefore something the user can
delete. In fact, we see this happen quite a lot, and there have been
some other discussions about it recently, see [1] and [2]. I bounced the
idea of a rename off various hackers at the conference and in general
people seemed to think it was a good idea.

Attached is a patch to rename backup_label to recovery_control. The
purpose is to make it more obvious that the file should not be deleted.
I'm open to other names, e.g. recovery.control. That makes the naming
distinct from tablespace_map, which is perhaps a good thing, but is also
more likely to be confused with recovery.signal.

I did a pretty straight-forward search and replace on comments and
documentation with only light editing. If this seems like a good idea
and we choose a final name, I'll do a more thorough pass through the
comments and docs to try and make the usage more consistent.

Note that there is one usage of backup label that remains, i.e. the text
that the user can set to describe the backup.

Regards,
-David

[1]
https://www.postgresql.org/message-id/1330cb48-4e47-03ca-f2fb-b144b49514d8%40pgmasters.net
[2]
https://www.postgresql.org/message-id/flat/CAM_vCudkSjr7NsNKSdjwtfAm9dbzepY6beZ5DP177POKy8%3D2aw%40mail.gmail.com#746e492bfcd2667635634f1477a61288