Split into 'unix' and 'xenctrl' subpackages, add configure script

Signed-off-by: David Scott <dave.scott@citrix.com>
Regenerate OASIS

Signed-off-by: David Scott <dave.scott@citrix.com>
opam: run configure

Signed-off-by: David Scott <dave.scott@citrix.com>
travis: add a test case without the Xen headers

Signed-off-by: David Scott <dave.scott@citrix.com>
Merge pull request #11 from djs55/refactor-modules

Try to fix build on OS X