Rewrite the Gnus group activation method to be more efficient.

Rewrite the Gnus group activation method to be more efficient.

The main idea is to gather all groups based on their select methods,
and then issuing one `-request-list' per select method, instead of
activating them group by group.  This should in theory be much, much
faster, but there's a bunch of subtle interactions between a plethora
of variables that doesn't make this trivial to achieve.

This is a first stab at the issue, and seems to work, but I'm sure
that there are corner cases that doesn't quite work right.