MB-4617 fork disksup to include mounts into the get_disk_data() result

...in order to have correct disk stats for mounted drives

the fork also includes OSX related code from R16 as a replacement for
our ns_info:get_disk_data() OSX patch. to be removed as soon as we
will permanently move to R16

Change-Id: I92a80791b11153074fd1640926ab61be324449b6
Reviewed-on: http://review.couchbase.org/37488
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>