bluetooth: introduce Bluetooth communication API, implement BlueZ support
Create a src/bt/ subdirectory for source files. Declare a platform
agnostic internal API for Bluetooth communication, and provide an
implementation of that portable API when the BlueZ library is available.
This implementation assumes that HAVE_BLUETOOTH and HAVE_LIBBLUEZ can be
used interchangeably, which is true for this initial version. When
support for other platforms gets added, the common and the specific
parts need to get sorted. Trying that now would involve guessing. :)