rmmod: behavior more like m-i-t's rmmod

rmmod: behavior more like m-i-t's rmmod

In line with m-i-t's behavior, we should check to see if each module is:

- loaded
- has any holders
- has a 0 refcnt

Detecting any of these lets us provide a more useful message than the
kernel's EPERM response to delete_module(2).

Additionally, alter the main loop behavior to avoid exiting early on the
first error.