Move afscell to platform/DARWIN

Move afscell to platform/DARWIN

Move our current afscell code into platform/DARWIN so that it's in a
location that can be added to the build, rather than packaging, step

Reviewed-on: http://gerrit.openafs.org/http://gerrit.openafs.org/200
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit b4564fa8254afbfef94915f1051a5ad7ab17b11d)
Change-Id: I2a13a556730cf2bd205a66609bc9cb34b12dcd47
Reviewed-on: http://gerrit.openafs.org/917