Minor bug with Group process mark

[Harry Putnam <reader@newsguy.com>]

This is something that has been around a very long time and only bites
me occasionally.

Many things can be done in emacs/gnus with process marks and in some
cases, once the action is taken the marks disappear.  Like for
example, when you process mark a number of messages and then say 
'/ n'.  You get a group limited to those process marked, but the mark 
has been removed in the limited view.

However, some processes don't remove the marks, leaving them there to
cause unexpected problems.

In group buffer if you process mark a number of groups and then sort
them with the `G P <whatever>' sort commands, once the the sort is
done the marks should disappear.  Its just a bit to easy to forget to
manually remove the marks, and do some other process on them all, by
accident, like maybe try to kill a group and kill all process marked
ones instead.

Shouldn't those marks disappear after the process is run?