Add ALTER SYSTEM command to edit the server configuration file.

Patch contributed by Amit Kapila. Reviewed by Hari Babu, Masao Fujii,
Boszormenyi Zoltan, Andres Freund, Greg Smith and others.

src/include/nodes/parsenodes.h

@@ -2471,6 +2471,16 @@ typedef struct DropdbStmt
 	bool		missing_ok;		/* skip error if db is missing? */
 } DropdbStmt;
 
+/* ----------------------
+ *		Alter System Statement
+ * ----------------------
+ */
+typedef struct AlterSystemStmt
+{
+	NodeTag		type;
+	VariableSetStmt *setstmt;	/* SET subcommand */
+}	AlterSystemStmt;
+
 /* ----------------------
  *		Cluster Statement (support pbrown's cluster index implementation)
  * ----------------------