Rename field "relkind" to "objtype" for CTAS and ALTER TABLE nodes

Rename field "relkind" to "objtype" for CTAS and ALTER TABLE nodes

"relkind" normally refers to the char field from pg_class.  However, in
the parse nodes AlterTableStmt and CreateTableAsStmt, "relkind" was used
for a field of type enum ObjectType, that could refer to other object
types than those possible for a relkind.  Such fields being usually
named "objtype", switch the name in both structures to make things more
consistent.  Note that this led to some confusion in functions that
also operate on a RangeTableEntry object, which also has a field named
"relkind".

This naming goes back to commit 09d4e96, where only OBJECT_TABLE and
OBJECT_INDEX were used.  This got extended later to use as well
OBJECT_TYPE with e440e12, not really a relation kind.

Author: Mark Dilger
Reviewed-by: Daniel Gustafsson, Álvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/609181AE-E399-47C7-9221-856E0F96BF93@enterprisedb.com