Adding a pg_get_owned_sequence function?

Hi hackers,

I've always been annoyed by the fact that pg_get_serial_sequence takes
the table and returns the sequence as strings rather than regclass. And
since identity columns were added, the name is misleading as well (which
is even acknowledged in the docs, together with a suggestion for a
better name).

So, instead of making excuses in the documentation, I thought why not
add a new function which addresses all of these issues, and document the
old one as a backward-compatibilty wrapper?

Please see the attached patch for my stab at this.

- ilmari