Compass: bug fix to init of device id

Also init members to reduce compiler warnings
Git: ignore Thumbs.db

These files can be created automatically on Windows machines and are
difficult to remove
AC_AttControl: cast fabs to float to resolve compiler warning
AHRS: initalise members to reduce compiler warnings
AC_AttControlHeli: cast fabs to float to resolve compiler warning
AC_AttControlHeli: remove hal declaration to clear compiler warning
AC_Circle: initialise members to reduce compiler warnings
AC_PosControl: cast fabs to float to resolve compiler warnings
AC_PosControl: init members to resolve compiler warnings
AP_InertialNav: append f to floating point constants

Reduces some compiler warnings
AP_InertialNav: init members to reduce compiler warnings
AC_Fence: append f to floating point constants
AP_InertialNav: remove virtual qualifier from error_count
Inav_EKF: init member and remove hal reference to reduce compiler warnings
Copter: add parenthesis around ntun log bitmask

Resolves one compiler warning
AC_Sprayer: append f to floating point constants

Reduces some compiler warnings
AP_Airspeed: use 'no break' to resolve compiler warning re switch
AP_Airspeed: init members to reduce compiler warnings
Camera: remove unused reference to hal

Resolves one compiler warning
AP_InertialSensor: init members to reduce compiler warnings