eal/bsd: support alarm API

eal/bsd: support alarm API

Implement EAL alarm API support for FreeBSD. The implementation
is largely identical to that of Linux version, with one key
difference.

The alarm API is a little Linux-centric in that it is expecting
the alarm API to manage alarm timeouts without involvement of the
interrupt thread. This works on Linux because in Linux, there's
timerfd API which allows waiting for timer events on an fd.

On FreeBSD, however, there are no timerfd's, and timer events are
set up directly in kevent. There is no way to pass information from
the alarm API to the interrupt thread, so we also add a little
back-channel magic to get soonest alarm timeout from the alarm API.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>