pg_get_serial_sequence

Hi,

Attached is a new backend function, pg_get_serial_sequence. It allows
you get a sequence name in a format suitable for use with the sequence
functions.

Has docs and regression.

I updated pg_dump to use it as well.

Tested against 7.5 and 7.4 backends, with independent and dependent
sequences. Handles quoting, escaping and schema-qualification.

Someone might want to run their eyes over my doc changes (I have no way
of compiling them) and check that my function is done in the most
efficient way. (It's small!)

Chris