Add bindings for some <os/*.h> APIs

[ronaldoussoren]

See also #376.

On first glance the follow headers are interesting enough:

• os/signpost.h
• os/log.h
• os/activity.h
• os/workgroup.h

Current plan is to add a new bindings package ("pyobjc-framework-libos") with one module per header (e.g. "import os_signpost").

A possible problem with a binding to os/log.h, and possibly os/signpost.h, is that the header documents that the format string argument must be a static string (no variable). This at the very least requires some functional tests to make sure that the regular PyObjC binding pattern works.