Change nextval and other sequence functions to specify their sequence

Change nextval and other sequence functions to specify their sequence
argument as a 'regclass' value instead of a text string.  The frontend
conversion of text string to pg_class OID is now encapsulated as an
implicitly-invocable coercion from text to regclass.  This provides
backwards compatibility to the old behavior when the sequence argument
is explicitly typed as 'text'.  When the argument is just an unadorned
literal string, it will be taken as 'regclass', which means that the
stored representation will be an OID.  This solves longstanding problems
with renaming sequences that are referenced in default expressions, as
well as new-in-8.1 problems with renaming such sequences' schemas or
moving them to another schema.  All per recent discussion.
Along the way, fix some rather serious problems in dbmirror's support
for mirroring sequence operations (int4 vs int8 confusion for instance).