Importing slightly modified virtio-net by vsrinivas

* TSO and LRO are off for now by default.

* Makefile changes. Adapting manpages. Fixing a typo in vtnet_tx_offload.

* Allocating tx headers with contigmalloc during attach instead of using
  kmalloc in the transmit path.
  In the worst case we need one tx header for two virtqueue ring entries.

* Additionally some whitespace fixes
vtnet.4: Add a reference to ifmedia(4).
kernel - Improve pid-reuse algorithm, fix bug

* Fix a bug where under extreme loads it was possible for a PID to be
  allocated twice.

* Implement a minimum pid-reuse delay of 10 seconds.  No pid, session id,
  or pgid will be reused for at least 10 seconds after being reaped.

  This shouldn't really be necessary but it should help scripts, particularly
  bulk builds, which rely on testing out-of-band PIDs with pwait.

* Increase PID_MAX from 99999 to 999999

Reported-by: marino