Tweak low velocity avatar keyboard control.

It used to be the case that as soon as you pressed a button the avatar would start at 1.5 m/sec.  This minimum speed has been reduced to half: 0.75m/sec.

Also took the opportunity to try to fix a bug that a user was reporting here:

    https://alphas.highfidelity.io/t/velocity-wich-velocity-i-lose-it-many-times/1010/4