Implement most effects of AGENT_CONTROL_STOP

AGENT_CONTROL_STOP is specified to SP.HandleAgentUpdate if the user holds down the space bar on a viewer.
For a stopped avatar, this prevents fly or walk/run (though not rotate) until released.
For a walking/running avatar, this reduces movement to half speed.
For a flying avatar, this stops the avatar.
These are observed behaviours on the LL grid - there was no previous OpenSimulator implementation
This commit introduces an optional parameter to SP.AddNewMovement(), which means that it will no longer compile on .NET 3.5 or earlier versions of Mono than 2.8
Currently, this does not work for jumping, and if used whilst flying the avatar continues the fly animation even though it does not move