Fix a bug where we would improperly calculate if the FIFO was

Fix a bug where we would improperly calculate if the FIFO was
full by adding a reserve feature to the packet fifo which allows
us to reserve space in the fifo if only part of a packet was
copied into the fifo.

dev/ns_gige.cc:
    use the new reserve feature in the fifo to properly determine
    when we're full.  assert that adding a packet to the fifo suceeds.
dev/pktfifo.hh:
    add the ability to reserve space in the fifo.  This is useful for
    partial writing of packets into the fifo.

--HG--
extra : convert_revision : 83f871f34fac237bb464c9513cf6490b5c62420e