Update SQL features count

The previously listed total of 179 does not appear to be correct for
SQL:2016 anymore.  (Previous SQL versions had slightly different
feature sets, so it's plausible that it was once correct.)  The
currently correct count is the number of rows in the respective tables
in appendix F in SQL parts 2 and 11, minus 2 features that are listed
twice.  Thus the correct count is currently 177.  This also matches
the number of Core entries the built documentation currently shows, so
it's internally consistent.

Author: petere

doc/src/sgml/features.sgml

@@ -78,8 +78,8 @@
 
  <para>
   PostgreSQL supports most of the major features of SQL:2016.  Out of
-  179 mandatory features required for full Core conformance,
-  PostgreSQL conforms to at least 160.  In addition, there is a long
+  177 mandatory features required for full Core conformance,
+  PostgreSQL conforms to at least 170.  In addition, there is a long
   list of supported optional features.  It might be worth noting that at
   the time of writing, no current version of any database management
   system claims full conformance to Core SQL:2016.