ocp-build make -doc and .ocpstop

* 'build' subcommand is replaced by 'make' subcommand
* For 'make' subcommand, new arguments:
   -build : build
   -doc : build and create documentation
   -test : build tests
* Use ocamldoc for "library" and "objects" to automatically build
    html documentation in _obuild/PACKAGE/doc/index.html.
* For "build", "doc" and "test", have equivalent "XXX_rules" and
   "XXX_targets" to define build rules and build targets
* ".ocpstop" in a directory prevents ocp-build from traversing the
   directory from above.