Improve castNode notation by introducing list-extraction-specific variants.

This extends the castNode() notation introduced by commit 5bcab1114 to
provide, in one step, extraction of a list cell's pointer and coercion to
a concrete node type.  For example, "lfirst_node(Foo, lc)" is the same
as "castNode(Foo, lfirst(lc))".  Almost half of the uses of castNode
that have appeared so far include a list extraction call, so this is
pretty widely useful, and it saves a few more keystrokes compared to the
old way.

As with the previous patch, back-patch the addition of these macros to
pg_list.h, so that the notation will be available when back-patching.

Patch by me, after an idea of Andrew Gierth's.

Discussion: https://postgr.es/m/14197.1491841216@sss.pgh.pa.us

src/backend/tcop/pquery.c

@@ -496,7 +496,7 @@ PortalStart(Portal portal, ParamListInfo params,
 				 * Create QueryDesc in portal's context; for the moment, set
 				 * the destination to DestNone.
 				 */
-				queryDesc = CreateQueryDesc(castNode(PlannedStmt, linitial(portal->stmts)),
+				queryDesc = CreateQueryDesc(linitial_node(PlannedStmt, portal->stmts),
 											portal->sourceText,
 											GetActiveSnapshot(),
 											InvalidSnapshot,
@@ -1036,7 +1036,7 @@ FillPortalStore(Portal portal, bool isTopLevel)
 			break;
 
 		case PORTAL_UTIL_SELECT:
-			PortalRunUtility(portal, castNode(PlannedStmt, linitial(portal->stmts)),
+			PortalRunUtility(portal, linitial_node(PlannedStmt, portal->stmts),
 							 isTopLevel, true, treceiver, completionTag);
 			break;
 
@@ -1232,7 +1232,7 @@ PortalRunMulti(Portal portal,
 	 */
 	foreach(stmtlist_item, portal->stmts)
 	{
-		PlannedStmt *pstmt = castNode(PlannedStmt, lfirst(stmtlist_item));
+		PlannedStmt *pstmt = lfirst_node(PlannedStmt, stmtlist_item);
 
 		/*
 		 * If we got a cancel signal in prior command, quit