Increase the (arbitrary) limit for the number of packets per tick
from 1k to 20k The previous value was good 10 years ago, but not
anymore now.

More importantly, lots of good surprises:
polling is incredibly effective under virtualization, and not only
prevents livelock but also saves most of the VM exit overhead in
receive mode.

Using polling, a FreeBSD instance under qemu-kvm remains perfectly
responsive even when bombed with 10 Mpps over an emulated e1000,
and happily processes 1.7 Mpps through ipfw.

Note that some incompatibilities still remain: e.g. polling is not
(yet) compatible with netmap, and seems to freeze the guest when
kern.polling.idle_poll=1

MFC after:	3 days
Remove reference to removed !MPSAFE filesystem support
Correct panic on detach of Xen PV network interfaces.

dev/xen/netfront:
    In netif_free(), properly stop the interface and drain any pending
    timers prior to disconnecting from the backend device.

    Remove all media and detach our interface object from the system
    prior to deleting it.

PR:		kern/176471
Submitted by:	Roger Pau Monne <roger.pau@citrix.com>
Reviewed by:	gibbs
MFC after:	1 week