Move abort() into opr

Move abort() into opr

We need our own abort function, because the behaviour of the Windows
abort() implementation isn't sufficiently flexible for us.
Because we're replacing an operating system function, reather than
implementing a missing function, this doesn't belong in roken.

So, provide an alternative opr_abort() implementation in our portable
runtime layer, which is a synonym for abort() on Unix, and implements
the required DebugBreak() functionality on Windows.

Remove lwp_abort() which was just creating another, unnecessary, layer
of abstraction.

Change-Id: Ice226d70d2791beaba011f42e39fde60242f6cc3
Reviewed-on: http://gerrit.openafs.org/4428
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>