Dump serials as serial

I intend to make 'serial' and 'serial8' domains of the int4 and int8
datatypes. Now they're regular types and columns in \d will be marked
as such. This leaves analyze.c to create the defaults but no longer
will it be doing any datatype conversions.

pg_dump and psql will be taught to ignore the default of 'serial' and
'serial8' datatypes. Perhaps a 'system_generated' flag on pg_attrdef
would be more appropriate?

We'd also be going with the assumption in pg_dump that a sequence name
will be generated the same way in future versions (see prior discussion
in hackers).