MFH: r461924 r462351 r462460

Use USE_GITHUB instead of hand crafting urls.

Sponsored by: Absolight

Add a NETMAP option to build and install the bro netmap plugin.

PR: 224918
Reported by: Shane Peters
Reviewed by: matthew (mentor)
Approved by: matthew (mentor)
Differential Revision: https://reviews.freebsd.org/D14378

Update to 2.5.3 which fixes an integer overflow:

http://blog.bro.org/2018/02/bro-253-released-security-update.html

Note that a CVE has not been assigned yet.

Reviewed by: matthew (mentor)
Approved by: matthew (mentor)
Differential Revision: https://reviews.freebsd.org/D14444

Approved by: ports-secteam (swills)