extensive orb_advert_t fixes

The calls to orb_advertise were being mishandled throughout the code.
There were ::close() calls on memory pointers, there were checks
against < 0 when it is a pointer to a object and values larger than
0x7ffffffff are valid. Some places orb_advert_t variables were
being initialized as 0 other places as -1.

The orb_advert_t type was changed to uintptr_t so the pointer value
would not be wrapped as a negative number. This was causing a failure
on ARM.

Tests for < 0 were changed to == 0 since a null pointer is the valid
representation for error, or uninitialized.

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

src/modules/land_detector/LandDetector.cpp

@@ -44,7 +44,7 @@
 #include <drivers/drv_hrt.h>
 
 LandDetector::LandDetector() :
-	_landDetectedPub(-1),
+	_landDetectedPub(0),
 	_landDetected({0, false}),
 	      _taskShouldExit(false),
 	      _taskIsRunning(false)
@@ -55,7 +55,6 @@ LandDetector::LandDetector() :
 LandDetector::~LandDetector()
 {
 	_taskShouldExit = true;
-	close(_landDetectedPub);
 }
 
 void LandDetector::shutdown()