Linux: Rework PAG to group conversions

Linux: Rework PAG to group conversions

The change to using a single group for PAGs on Linux led to a load
of special cases throughout the code. This patch moves the OS
dependent parts of this into the LINUX/ platform directory, and
rearranges that file so that there is a clear distinction between
the one and two group sections of the code.

Change-Id: Ib6e6f081519b23494dcf731b4be689c006180c68
Reviewed-on: http://gerrit.openafs.org/1007
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>