Change backend._OSSearch return values

Currently, the function backend._OSSearch() returns the (first) base dir
in which this OS can be found. Thereafter the full actual path to the OS
dir is built in the backend.OSFromDisk() function.

This patch changes this so that _OSSearch() always returns the full path
to the OS directory, and OSFromDisk uses that as returned (it will only
build it if it gets a base dir in the first place).

This patch is needed before we can abstract the _OSSearch into a generic
'look for file object' functionality that can be used for allocator
plugins search too.

Reviewed-by: ultrotter