Have pg_dumpall write CREATE USER and CREATE GROUP commands instead of raw

COPYs of pg_shadow and pg_group.

It also turns out that pg_dumpall was all but broken for multiple servers
running at non-standard port numbers. You might get the users and groups
from one server and the databases from another. Fixed that.

A little user interface and code cleanup along with that. This also takes
care of the portability bug discussed in "[BUGS] pg_dumpall" in March 2000.