Fix losetup so that it A) actually works again, B) has much better error

Fix losetup so that it A) actually works again, B) has much better error
messages, C) can show the current association (if any) when called
with only one argument.  Update the documentation a lot too.

Remind me to add a test suite for this thing.  I think I've figured out
how to handle root-only testsuites...