AP_Common: make alt in Location 24 bit

this allows for up to 83km mission altitudes, while allowing for 1
more byte in the command structure, making p1 16 bits
AP_Mission: make p1 16 bit

this gives us a 16 bit value for extra data on a location, allowing
for an accurate angle in centi-degrees.

Also add validation of the size of the union arms in Content