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/utils/misc/guc.c

@@ -7366,7 +7366,7 @@ ExecSetVariableStmt(VariableSetStmt *stmt, bool isTopLevel)
 			}
 			else if (strcmp(stmt->name, "TRANSACTION SNAPSHOT") == 0)
 			{
-				A_Const    *con = castNode(A_Const, linitial(stmt->args));
+				A_Const    *con = linitial_node(A_Const, stmt->args);
 
 				if (stmt->is_local)
 					ereport(ERROR,