Bump revision to 2.1.0, since the addition of a MacOS X interface
can cause pattern matching failures in config.ml files
Remove `--no-macosx` and add a `--target` enum

This accepts `macosx`,`unix`, or `xen`, and the old `--xen` and
`--unix` will continue to work.  Also improve the docstrings to
cmdliner to explain the new behaviour better.
Merge pull request #336 from avsm/2.1-bump

Bump to 2.1.0 and add `--target` instead of `--no-macosx`