Add the capability to iterate through the packets in a pktfifo,

Add the capability to iterate through the packets in a pktfifo,
and to remove elements in the middle of the fifo.  These elements
do not free space, they are just marked removed.  Space is only
freed from the front of the fifo.

dev/etherpkt.cc:
    serialize the current slack
dev/etherpkt.hh:
    add "slack" to the ethernet packet.  It is to be used by any fifo that
    the packet is currently in to account for extra space that the packet
    may be occupying due to the fifo organization.

--HG--
extra : convert_revision : 8e7c541ba316a9a76495c54cc5f707f8fc65b6d5