new port sysutils/circonus-agent: agent for metric collection integrating with the circonus framework.

Submitted by: sam.gwydir@joyent.com
Reviewed by: swills (mentor), tobik/mat (earlier versions)
Approved by: swills (mentor)
Obtained from: D13365 (with changes)
Differential Revision: https://reviews.freebsd.org/D13365