PATCH: Expose generate_qualified_relation_name functionality

I'm regularly surprised that the only non-static function we seem to
have for getting a relation name by oid is get_rel_name. It doesn't
handle schema qualification at all, and it returns NULL rather than
ERROR'ing.

Doing it correctly involves interacting with the syscache, calling
RelationIsVisible and calling get_namespace_name if needed, then
passing the result to quote_qualified_identifier.

so here's what I propose to do:

Add get_qualified_relation_name(bool force_qualified, bool missing_ok)
to ruleutils.c and builtins.h.

(Unless there's somewhere better? I wanted to put it in lsyscache.c
alongside get_rel_name, but it uses quote_qualified_identifier,
StringInfo, etc, so it doesn't fit well in lsyscache.c.)

Replace generate_qualified_relation_name in ruleutils.c with a call to
get_qualified_relation_name(relid, true, false) .

Add comments to RelationGetRelationName, get_rel_name and regclassout
pointing to get_qualified_relation_name.

The only part I don't like here is the two boolean arguments, since
they're ugly to read. But inventing a new flags field seemed a bit
heavy for such a trivial task.

I had a quick look through the codebase for places where this pattern
is repeated and found astonishingly few. In most places we just use
get_rel_name and hope the user can figure out any ambiguity.

I did notice that in heap_copy_data and AlterTableMoveAll we manually
schema-qualify a relation name and fail to call
quote_qualified_identifier, so it's unclear if we mean a relation
named "my.relation" or the relation "relation" in schema "my". But in
diagnostic output, meh, whatever.

Arguably regclassout does the same thing as
get_qualified_relation_name, but I didn't replace it because it cares
about IsBootstrapProcessingMode().

(Prompted by https://dba.stackexchange.com/q/187788/7788)

--
Craig Ringer http://www.2ndQuadrant.com/
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