Remove returning_checking_errno; pass a flag to foreign instead.
Merge pull request #72 from yallop/check-errno-in-foreign

Remove returning_checking_errno; pass a flag to foreign instead.
"Abstract" values may be passable in C but not supported by libffi.

We need this to support the caml 'value' type, which is treated as abstract.
C syntax (WIP).
C stub generation (WIP).
Beginnings of analysis of generated C (WIP).