Dump public schema ownership & seclabels

https://postgr.es/m/20201031163518.GB4039133@rfd.leadboat.com gave $SUBJECT as
one of the constituent projects for changing the public schema default ACL.
This ended up being simple.  Attached.  I chose to omit the "ALTER SCHEMA
public OWNER TO" when the owner is the bootstrap superuser, like how we skip
acl GRANT/REVOKE when the ACL matches the one recorded in pg_init_privs.  I
waffled on that; would it be better to make the OWNER TO unconditional?

Like ownership, we've not been dumping security labels on the public schema.
The way I fixed ownership fixed security labels implicitly.  If anyone thinks
I should unbundle these two, let me know.

All this is arguably a fix for an ancient bug.  Some sites may need to
compensate for the behavior change, so I plan not to back-patch.

Thanks,
nm