Add support for parent monitor for unix platforms

Let memcached look for the availability for the parent process
and kill itself if its gone. This piece of code is only used by
the unit tests to ensure that we don't have zombie memcached
processes running if the unit tests fail.

Change-Id: I462af679d7691a1897b46448e9dc6a2613a56372
Reviewed-on: http://review.couchbase.org/33170
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>