Make all network interrupt service routines MPSAFE part 1/3.

Make all network interrupt service routines MPSAFE part 1/3.

Replace the critical section that was previously used to serialize access
with the LWKT serializer.  Integrate the serializer into the IFNET structure.
Note that kern.intr_mpsafe must be set to 1 for network interrupts to actually
run MPSAFE.  Also note that any interrupts shared with othre non-MP drivers
will cause all drivers on that interrupt to run with the Big Giant Lock.

Network interrupt - Each network driver then simply passes that serializer
to bus_setup_intr() so only a single serializer is required to process the
entire interrupt path.  LWKT serialization support is already 100% integrated
into the interrupt subsystem so it will already be held as of when the
registered interrupt procedure is called.

Ioctl and if_* functions - All callers of if_* functions (such as if_start,
if_ioctl, etc) now obtain the IFNET serializer before making the call.  Thus
all of these entry points into the driver will now be serialized.

if_input - All code that calls if_input now ensures that the serializer
is held.  It will either already be held (when called from a driver), or
the serializer will be wrapped around the call.  When packets are forwarded
or bridged between interfaces, the target interface serializer will be
dropped temporarily to avoid a deadlock.

Device Driver access - dev_* entry points into certain pseudo-network
devices now obtain and release the serializer.  This had to be done on
a device-by-device basis (but there are only a few such devices).

Thanks to several people for helping test the patch, in particular
Sepherosa Ziehau.