debugnet(4): Infer non-server connection parameters

Loosen requirements for connecting to debugnet-type servers. Only require a
destination address; the rest can theoretically be inferred from the routing
table.

Relax corresponding constraints in netdump(4) and move ifp validation to
debugnet connection time.

Submitted by: John Reimer <john.reimer AT emc.com> (earlier version)
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D21482