Remove useless code that propagated FrontendProtocol to a backend via a

PostgresMain switch.  In point of fact, FrontendProtocol is already set
in a backend process, since ProcessStartupPacket() is executed inside
the backend --- it hasn't been run by the postmaster for many years.
And if it were, we'd still certainly want FrontendProtocol to be set before
we get as far as PostgresMain, so that startup errors get reported in the
right protocol.

-v might have some future use in standalone backends, so I didn't go so
far as to remove the switch outright.

Also, initialize FrontendProtocol to 0 not PG_PROTOCOL_LATEST.  The only
likely result of presetting it like that is to mask failure-to-set-it
mistakes.

src/backend/tcop/postgres.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/tcop/postgres.c,v 1.569 2009/07/31 20:26:23 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/tcop/postgres.c,v 1.570 2009/08/28 18:23:53 tgl Exp $
  *
  * NOTES
  *	  this is the "main" module of the postgres backend and
@@ -3075,6 +3075,13 @@ PostgresMain(int argc, char *argv[], const char *username)
 				}
 
 			case 'v':
+				/*
+				 * -v is no longer used in normal operation, since
+				 * FrontendProtocol is already set before we get here.
+				 * We keep the switch only for possible use in standalone
+				 * operation, in case we ever support using normal FE/BE
+				 * protocol with a standalone backend.
+				 */
 				if (secure)
 					FrontendProtocol = (ProtocolVersion) atoi(optarg);
 				break;