Fix underqualified cast-target type names in pg_dump and psql queries.

Queries running with some non-pg_catalog schema frontmost in their search
path need to be careful to schema-qualify type names that should be sought
in pg_catalog.  Vitaly Burovoy reported an oversight of this sort in
pg_dump's dumpSequence, and grepping detected another one in psql's
describeOneTableDetails, both introduced by sequence-related changes in
v10.  In pg_dump, we can fix things by removing the cast altogether, since
it doesn't really matter what data types are reported for these query
result columns.  Likewise in psql, the query seemed to be working unduly
hard to get a result that's guaranteed to be exactly 'bigint'.

I also changed a couple of occurrences of "::char" similarly.  These are
not bugs, since "char" is a typename keyword and not subject to search_path
rules, but it seems better to use uniform style.

Vitaly Burovoy and Tom Lane

Discussion: https://postgr.es/m/CAKOSWN=ds66zLw2SqkLTM8wbXFgDbc_OdkmT3dJfPT2mE5kipA@mail.gmail.com