afsocket: added support SO_REUSEPORT_LB

[drTr0jan]

Added support of SO_REUSEPORT_LB socket option, allowing multiple programs or threads to bind to the same port, and incoming connections load balanced using a hash function. The option has been included in FreeBSD 12.0 (freebsd/freebsd-src@bbf7d4d).

As continuation for #2379.

[kira-syslogng]

This user does not have permission to start the build. Can one of the admins verify this patch and start the build?
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[kira-syslogng]

This user does not have permission to start the build. Can one of the admins verify this patch and start the build?
(admin: you have the next options (make sure you checked the code):
"ok to test" to accept this pull request (and further changes) for testing
"test this please" for a one time test run
"add to whitelist" add author of a Pull Request to whitelist (globally, be careful, it means this user can trigger kira for any PR)
do nothing -> CI won't start)

[kira-syslogng]

This user does not have permission to start the build. Can one of the admins verify this patch and start the build?
(admin: you have the next options (make sure you checked the code):
"ok to test" to accept this pull request (and further changes) for testing
"test this please" for a one time test run
"add to whitelist" add author of a Pull Request to whitelist (globally, be careful, it means this user can trigger kira for any PR)
do nothing -> CI won't start)

[furiel]

@kira-syslogng ok to test

[kira-syslogng]

Build SUCCESS

[gaborznagy]

Thanks for the contribution! 👍