fragmentation: Fix fragmentation field check

When an incoming packet is for secnet itself, secnet checks the
fragmentation field in the IP header.

(Contrary to the spec, secnet discards fragmented packets addressed to
its own private address; however, this is a tolerable defect as secnet
never sends packets of its own apart from ICMP errors and ICMP Echo
Reply.)

However, secnet would incorrectly check the reserved flag bit in the
16-bit fragmentation superfield.  Fix this.  Also introduce some
manifest constants for the bits in the 16-bit fragmentation
superfield.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>