mc_pos_control: Add parameter support for terrain hold function
mc_pos_control: Add parameter support for terrain hold function

This new mode of altitude control uses terrain following when holding position and normal altitude control when moving.

PX4/Firmware,Aerotenna/Firmware,dagar/Firmware,Aerotenna/Firmware,Aerotenna/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,Aerotenna/Firmware,krbeverx/Firmware,dagar/Firmware,acfloria/Firmware,acfloria/Firmware,dagar/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,dagar/Firmware,acfloria/Firmware,PX4/Firmware,krbeverx/Firmware,krbeverx/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,Aerotenna/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,PX4/Firmware,krbeverx/Firmware,Aerotenna/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,Aerotenna/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,PX4/Firmware