Enable Doxygen option MULTILINE_CPP_IS_BRIEF, so that we don't have to
use \brief: C++-style comments are for brief descriptions and C-style
comments for extended descriptions.
Fix some comments (use @param instead of @arg).
To be consistent with the other modules, the positioning server must be
called OwlPS Positioner instead of Positioning, and the executable
owlps-positionerd (even if the daemon mode is not implemented yet).
That's a big commit for a tiny change.