Add port ftp/netdumpd.

This package contains a daemon which is used to receive kernel dumps over
a network from a panicking kernel. The protocol uses UDP and is similar to
TFTP.

Client support for FreeBSD is currently in progress.

Reviewed by: bdrewery
Approved by: bdrewery
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D11165