The new LioCb structure allows us to control the exact arguments passed
to lio_listio, guaranteeing that each call gets a unique storage
location for the list argument.  This prevents clients from misusing
lio_listio in a way that causes events to get dropped from a kqueue

Fixes nix-rust#870

A double panic can screw up the first panic's stack trace.  Better not
to assert! anything when the thread is already panicing.

Supporting the bytes crate was unnecessarily specific.  This change
replaces from_bytes and from_bytes_mut with from_boxed_slice and
from_boxed_mut_slice, which can work with anything that implements
Borrow<[u8]> and BorrowMut<[u8]>, respectively.

It helps deal with errors like EAGAIN, which can result in a subset of
an LioCb's operations being queued.  The test is only enabled on
FreeBSD, because it requires intimate knowledge of AIO system limits.