renice.el version 2

[Kevin Ryde]

I made an M-x renice which uses the "renice" program to decrease
priority of a sub-process in Emacs.

    http://user42.tuxfamily.org/renice/index.html

I use it to lower a compilation-mode program which is taking a long
time, so it's below other jobs.  Also works in an M-x list-processes.
Not much more than run "renice", but picks out pid for process, and
current priority is shown if possible.