This function calculated everything correctly but ultimately set the vertical velocity wrong. Most importantly this meant that the actual velocity vector and actor pitch - if CMF_SAVEPITCH was used - did not match.
Since this bug has been present since the pitch parameter was added, this deprecates A_CustomMissile and replaces it with a properly implemented A_SpawnProjectile function and handling the compatibility case with a new flag and a scripted wrapper function.
All internal uses of A_CustomMissile have been replaced as well.
