core: introduce (and fix) code to pass errno to other OSes

core: introduce (and fix) code to pass errno to other OSes

 - Some of the errno conversions were lost in the conversion
   to Cmake config
 - rename ceph_to_host_errno to _ceph_to_hostos_errno
   to indicate that the errnos are define per OS.

This is the second part, creating the basics.
Next it needs to be glued into communication with the peers.

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>