[New Port] sysutils/mping

mping is a utility for pinging multiple hosts. It is written in shell with
minimal dependencies (awk, date, getconf, head, host, ping, sleep, sort, stty,
tr, uname, and xargs). It also has a feature to keep watching the given hosts
and alert to any changes in uptime.

Reviewed by: mat (mentor)
Approved by: mat (mentor)
Sponsored by: Smule, Inc.
Differential Revision: https://reviews.freebsd.org/D15875