ip: reassembly - add a way to disable for forus

Add API to disable full reassembly of "forus" packets. Mark packets
passing through ip[4|6]-local nodes with a new buffer flag and check for
that flag in reassembly.

Enable IP6 "forus" full reassembly by default to be consistent with
existing IP4 setting.

Type: improvement
Change-Id: I7067792fcd4304182654237968e4c4d9293c6143
Signed-off-by: Klement Sekera <ksekera@cisco.com>