Set pg_setting.pending_restart when pertinent config lines are removed

This changes the behavior of examining the pg_file_settings view after
changing a config option that requires restart.  The user needs to know
that any change of such options does not take effect until a restart,
and this worked correctly if the line is edited without removing it.
However, for the case where the line is removed altogether, the flag
doesn't get set, because a flag was only set in set_config_option, but
that's not called for lines removed.  Repair.

(Ref.: commits 62d16c7fc5 and a486e35706)

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/202107262302.xsfdfc5sb7sh@alvherre.pgsql

src/backend/utils/misc/guc-file.l

@@ -347,6 +347,8 @@ ProcessConfigFileInternal(GucContext context, bool applySettings, int elevel)
 			continue;
 		if (gconf->context < PGC_SIGHUP)
 		{
+			/* The removal can't be effective without a restart */
+			gconf->status |= GUC_PENDING_RESTART;
 			ereport(elevel,
 					(errcode(ERRCODE_CANT_CHANGE_RUNTIME_PARAM),
 					 errmsg("parameter \"%s\" cannot be changed without restarting the server",