Add netdump support to dumpon(8).

A new usage is added so that parameters for netdump may be specified.
Specifically, one configures an interface for netdump with:

dumpon -c <client IP> -s <server IP> [-g <gateway IP>] <iface name>

Reviewed by: bdrewery, cem (earlier versions), sbruno
MFC after: 1 month
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D15254