Merge pull request #80 from avsm/master

fixes for 1.0: quieten warnings and cleaner output
Remove the Unix C binding (just `checksum` stubs) as not necessary.

They will move into `mirage-tcpip-unix` instead, and the Xen version
will remain embedded into the runtime.  This saves having to install
a dynamically loadable C library for mirage-unix, which leads to
issues such as mirage/mirage#130