Avoid superuser() from FTS message handler process.

FTS message handler process is special system process, which doesn't
have access to catalog. Hence, it shouldn't be performing any catalog
access. Calling `superuser()` via `set_gp_replication_config()` or
`pg_reload_conf()` results in error for the same reason. Hence, if
`am_ftshandler` process avoid calling `superuser()` and thereby avoid
catalog access. Rest all the stuff performed by this process doesn't
need catalog access.

Fixes #4764 github issue.

src/backend/utils/adt/misc.c

@@ -28,6 +28,7 @@
 #include "funcapi.h"
 #include "miscadmin.h"
 #include "parser/keywords.h"
+#include "postmaster/fts.h"
 #include "postmaster/syslogger.h"
 #include "rewrite/rewriteHandler.h"
 #include "storage/fd.h"
@@ -225,7 +226,7 @@ pg_terminate_backend_msg(PG_FUNCTION_ARGS)
 Datum
 pg_reload_conf(PG_FUNCTION_ARGS)
 {
-	if (!superuser())
+	if (!am_ftshandler && !superuser())
 		ereport(ERROR,
 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
 				 (errmsg("must be superuser to signal the postmaster"))));

src/backend/utils/misc/guc.c

@@ -57,6 +57,7 @@
 #include "postmaster/autovacuum.h"
 #include "postmaster/bgworker.h"
 #include "postmaster/bgwriter.h"
+#include "postmaster/fts.h"
 #include "postmaster/postmaster.h"
 #include "postmaster/syslogger.h"
 #include "postmaster/walwriter.h"
@@ -6998,7 +6999,7 @@ AlterSystemSetConfigFile(AlterSystemStmt *altersysstmt)
 	struct stat st;
 	void	   *newextra = NULL;
 
-	if (!superuser())
+	if (!am_ftshandler && !superuser())
 		ereport(ERROR,
 				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
 			 (errmsg("must be superuser to execute ALTER SYSTEM command"))));