Copter: use relative_alt flag when decoding change_alt command
AP_Common: remove unused location mask definitions
AP_Common: add LOWBYTE, HIGHBYTE macros
AP_Mission: squeeze loiter turns radius into high byte of p1
AP_Common: add example sketch
Copter: loiter turns radius from command's p1 field
Plane: loiter num turns from low byte of p1

This limits the maximum number of turns to 255 for plane