Extend the pt_entry api to handle flowspec.

Extend the pt_entry api to handle flowspec.
Introduce pt_get_flow() and pt_add_flow() to lookup and insert flowspec
objects. Add pt_getflowspec() which works somewhat similar to pt_getaddr()
to extract the flowspec NLRI from a pt_entry.
Make pt_getaddr() to return the destination prefix of the flowspec rule and
handle flowspec in pt_write().
OK tb@