Almost all of SpecGenToOCI deals with linux-specific aspects of the
runtime spec. Rather than try to factor this out piecemeal, I think it
is cleaner to move the whole function along with its implementation
helper functions. This also meams we don't need non-linux stubs for
functions called from oci_linux.go
[NO NEW TESTS NEEDED]
Signed-off-by: Doug Rabson <dfr@rabson.org>