Prevent running pg_resetwal/pg_resetxlog against wrong-version data dirs.

Prevent running pg_resetwal/pg_resetxlog against wrong-version data dirs.

pg_resetwal (formerly pg_resetxlog) doesn't insist on finding a matching
version number in pg_control, and that seems like an important thing to
preserve since recovering from corrupt pg_control is a prime reason to
need to run it.  However, that means you can try to run it against a
data directory of a different major version, which is at best useless
and at worst disastrous.  So as to provide some protection against that
type of pilot error, inspect PG_VERSION at startup and refuse to do
anything if it doesn't match.  PG_VERSION is read-only after initdb,
so it's unlikely to get corrupted, and even if it were corrupted it would
be easy to fix by hand.

This hazard has been there all along, so back-patch to all supported
branches.

Michael Paquier, with some kibitzing by me

Discussion: https://postgr.es/m/f4b8eb91-b934-8a0d-b3cc-68f06e2279d1@enterprisedb.com