PR#6060: Add some missing compiler options to ocamlbuild. (Patch by Anil Madhavapeddy!)

* -short-paths is activated via the "short_paths" tag
* -principal is activated via the "principal" tag
* -strict_sequence now has a "strict_sequence" tag to alias the
  "strict-sequence" tag that was already there, to follow the
  convention of command-line options having dashes replaced by
  underscores.  It's easy to mess this up since incorrect tags
  are silently ignored by ocamlbuild.

Add test cases that check if principal and strict-sequence have
been passed, and tweak the test suite slightly to make it easier
to match on failing_msg output.

git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13859 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02