AP_GPS: added explicit cast to omit [-Wconversion] warning
Copter: fixed a compiler warning

* removed useless comparison: if (int8_t t >= 255) ...
* revealed by -Wtype-limits
* saved 6 bytes
AP_Compass: removed comparison out of range compiler warning

uint_ >= 0 (-Wtype-limits)
AP_Param: fixed some compiler warnings

* added comment
* replaced "~0" with the appropriate macro
* resolved -Wconversion
* saved some space by modifying AP_Param::add_vector_suffix (writing
behind buffer could happen, if buffer_size is almost uint16_max but this
can surely be ignored)

further reading:
http://stackoverflow.com/questions/809227/is-it-safe-to-use-1-to-set-all-bits-to-true
AP_Param: fixed a compiler warning with -Wsign-conversion