uORB: Major refactoring

uORB was refactored in order to support the MuORB changes required
for QURT. Those changes wil be added in a subsequent commit.

The files are split out by posix and nuttx so the changes are visible.
When this has been tested, the files can be re-merged and re-tested.

Signed-off-by: Mark Charlebois <charlebm@gmail.com>