We had some pretty ad-hoc handling of the public schema and the plpgsql
extension, which are both presumed to exist in template0 but might be
modified or deleted in the database being dumped.

Up to now, by default pg_dump would emit a CREATE EXTENSION IF NOT EXISTS
command as well as a COMMENT command for plpgsql.  The usefulness of the
former is questionable, and the latter caused annoying errors in
non-superuser dump/restore scenarios.  Let's instead install a rule that
built-in extensions (identified by having low-numbered OIDs) are not to be
dumped.  We were doing it that way already in binary-upgrade mode, so this
just makes regular mode behave the same.  It remains true that if someone
has installed a non-default ACL on the plpgsql language, that will get
dumped thanks to the pg_init_privs mechanism.  This is more consistent with
the handling of built-in objects of other kinds.

Also, change the very ad-hoc mechanism that was used to avoid dumping
creation and comment commands for the public schema.  Instead of hardwiring
a test in _printTocEntry(), make use of the DUMP_COMPONENT_ infrastructure
to mark that schema up-front about what we want to do with it.  This has
the visible effect that the public schema won't be mentioned in the output
at all, except for updating its ACL if it has a non-default ACL.
Previously, while it was normally not mentioned, --clean mode would drop
and recreate it, again causing headaches for non-superuser usage.  This
change likewise makes the public schema less special and more like other
built-in objects.

If plpgsql, or the public schema, has been removed entirely in the source
DB, that situation won't be reproduced in the destination ... but that
was true before.

Discussion: https://postgr.es/m/29048.1516812451@sss.pgh.pa.us