Allow CREATE TABLE IF EXIST so succeed if the schema is nonexistent

Previously, CREATE TABLE IF EXIST threw an error if the schema was
nonexistent.  This was done by passing 'missing_ok' to the function that
looks up the schema oid.
Issue ERROR if FREEZE mode can't be honored by COPY

Previously non-honored FREEZE mode was ignored.  This also issues an
appropriate error message based on the cause of the failure, per
suggestion from Tom.  Additional regression test case added.
Update LookupExplicitNamespace() comments;  commit message update

Also, commit 7e2322dff30c04d90c0602d2b5ae24b4881db88b affected DROP
TABLE IF EXISTS, not CREATE TABLE IF EXISTS.
Update comments in new DROP IF EXISTS code; commit message update

DROP IF EXISTS with a missing schema in commit
7e2322dff30c04d90c0602d2b5ae24b4881db88b applies not only to tables, but
to DROP IF EXISTS with missing schemas for indexes, views, sequences,
and foreign tables.  Yeah!
Make LATERAL implicit for functions in FROM.

The SQL standard does not have general functions-in-FROM, but it does
allow UNNEST() there (see the <collection derived table> production),
and the semantics of that are defined to include lateral references.
So spec compliance requires allowing lateral references within UNNEST()
even without an explicit LATERAL keyword.  Rather than making UNNEST()
a special case, it seems best to extend this flexibility to any
function-in-FROM.  We'll still allow LATERAL to be written explicitly
for clarity's sake, but it's now a noise word in this context.

In theory this change could result in a change in behavior of existing
queries, by allowing what had been an outer reference in a function-in-FROM
to be captured by an earlier FROM-item at the same level.  However, all
pre-9.3 PG releases have a bug that causes them to match variable
references to earlier FROM-items in preference to outer references (and
then throw an error).  So no previously-working query could contain the
type of ambiguity that would risk a change of behavior.

Per a suggestion from Andrew Gierth, though I didn't use his patch.