Tested on macOS 10.11 dtrace, Ubuntu 16.04 SystemTap, and libbcc.

Largely based by an initial patch by Jesús Cea Avión, with some
influence from Dave Malcolm's SystemTap patch and Nikhil Benesch's
unification patch.

Things deliberately left out for simplicity:
- ustack helpers, I have no way of testing them at this point since
they are Solaris-specific
- PyFrameObject * in function__entry/function__return, this is
SystemTap-specific
- SPARC support
- dynamic tracing
- sys module dtrace facility introspection

All of those might be added later.

This is required by RFC 1542, so despite the subtle behavior change we
are treating it as a bug.  Patch by Michael Lazar.

pretty early on in the 3.x series (3.1 or 3.2?).

Builds on non-OSX UNIX now default to using the system libffi, and warn if the
bundled copy is used.