Fix handling of ALTER DEFAULT PRIVILEGES

Greetings,

While working through some pg_dump regression tests, I came across a bug
in 9.6+ due to the changes I made to how GRANT/REVOKEs were handled.

The attached describes and fixes the bug, which only impacts dumping
from 9.6+ clusters, so hopefully not too many people have been impacted
by it (it's also a bit of an odd case anyway, as noted).

I'm still reviewing and playing with it, but anticipate pushing it
sometime tomorrow, along with a bunch of additional pg_dump regression
tests, built on the prior framework (as an independent patch, and just
for PG10, whereas the attached will be back-patched to 9.6).

Thoughts and comments welcome, of course, but it's a relatively
mechanical set of changes to match how other the ACLs for everything
else are handled (I checked all other callers of buildACLCommands()).

Thanks!

Stephen