Allow SET TABLESPACE to database default

We've always allowed CREATE TABLE to create tables in the database's default
tablespace without checking for CREATE permissions on that tablespace.
Unfortunately, the original implementation of ALTER TABLE ... SET TABLESPACE
didn't pick up on that exception.

This changes ALTER TABLE ... SET TABLESPACE to allow the database's default
tablespace without checking for CREATE rights on that tablespace, just as
CREATE TABLE works today.  Users could always do this through a series of
commands (CREATE TABLE ... AS SELECT * FROM ...; DROP TABLE ...; etc), so
let's fix the oversight in SET TABLESPACE's original implementation.
Allow type_func_name_keywords in even more places

A while back, 2c92edad48796119c83d7dbe6c33425d1924626d allowed
type_func_name_keywords to be used in more places, including role
identifiers.  Unfortunately, that commit missed out on cases where
name_list was used for lists-of-roles, eg: for DROP ROLE.  This
resulted in the unfortunate situation that you could CREATE a role
with a type_func_name_keywords-allowed identifier, but not DROP it
(directly- ALTER could be used to rename it to something which
could be DROP'd).

This extends allowing type_func_name_keywords to places where role
lists can be used.

Back-patch to 9.0, as 2c92edad48796119c83d7dbe6c33425d1924626d was.
Fix bugs in PQhost().

In the platform that doesn't support Unix-domain socket, when
neither host nor hostaddr are specified, the default host
'localhost' is used to connect to the server and PQhost() must
return that, but it didn't. This patch fixes PQhost() so that
it returns the default host in that case.

Also this patch fixes PQhost() so that it doesn't return
Unix-domain socket directory path in the platform that doesn't
support Unix-domain socket.

Back-patch to all supported versions.