FOH mode for altitude

This introduces a parameter for the navigator. When enabled the navigator
publishes a first order old (FOH) type altitude setpoint instead of the
default zero order hold. For takeoff and landing the FOH mode is not
active. The FOH altitude is calculated such that the sp reaches the
altitude of the waypoint when the system is at a horizontal distance
equal to the acceptance radius. Also the altitude setpoint will only
converge towards the waypoint altitude but never diverge.
whitespace
foh alt mode: never sink below previous wp alt
Merge pull request #1349 from PX4/fohaltitude

FOH mode for altitude (optional, enable with parameter)