Cruise : improve behaviour from Benoit's comments

- Add a decrease factor that allows decrease speed faster than we
increse it.
Default value is :
DECREASE_RATE_FACTOR=2

- Allows negative speed in cruise mode
Added a new param to set this maximum backward speed:
CR_MAX_BW = 2km/h by default

Each of these defines and params MUST HAVE POSITIVE VALUE!
Mind to check in full param list the params are properly initialised
with default values:
WPNAV_CR_POT_RNG = 27000
WPNAV_CR_MAX_FW = 166.7  (6km/h)
WPNAV_CR_MAX_BW = 55.56  (2km/h)
WPNAV_CR_VEL_INC = 55.56   (2km/h/s full stick)