ip: implement output packet fragmentation

Add an ip_fragment node that fragments IPv4 packets exceeding the
outgoing interface MTU. The node is invoked from ip_output when
a packet is too large and does not have the DF (Don't Fragment) flag
set.

Fragmentation follows RFC 791 requirements: each fragment payload size
is rounded down to a multiple of 8 bytes, fragment offset field is
expressed in 8-byte units, and the MF (More Fragments) flag is set on
all fragments except the last one. All fragments share the same packet
ID from the original packet.

The ip_output node now stores the output interface in mbuf metadata
before the size check, ensuring ip_fragment can access the MTU for
proper fragment sizing. The previous ip_output_too_big drop node is
removed as oversized packets are now handled by ip_fragment.

Fragments inherit trace state from the original packet to maintain
debugging capability across fragmentation boundaries.

Closes: #336
Signed-off-by: Anthony Harivel <[email protected]>
Reviewed-by: Christophe Fontaine <[email protected]>

Author: aharivel