psql autocompletion for \z and \dg

Hi all,

It looks like psql's tab completion for the \z and \dg commands in
psql are missing. I couldn't see a reason for this, so attached patch
fixes.

Also, this patch proposes to change psql's "\?" help text to say that
\dg and \du are the same, since AFAICT they do exactly the same thing.

Josh