Advance PG_CONTROL_VERSION.

This should have been done in commit 73c986adde which added several
new fields to pg_control, and again in commit 5028f22f6e which
changed the CRC algorithm, but it wasn't.  It's far too late to fix it in
the 9.5 branch, but let's do so in 9.6, so that if a 9.6 postmaster is
started against a 9.4-era pg_control it will complain about a versioning
problem rather than a CRC failure.  We already forced initdb/pg_upgrade
for beta3, so there's no downside to doing this now.

Discussion: <7615.1468598094@sss.pgh.pa.us>

src/include/catalog/pg_control.h

@@ -21,7 +21,7 @@
 
 
 /* Version identifier for this pg_control format */
-#define PG_CONTROL_VERSION	942
+#define PG_CONTROL_VERSION	960
 
 /*
  * Body of CheckPoint XLOG records.  This is declared here because we keep