Updated the reverted fix for critical bug EXT-1655 ([BSI] Always flying after...

Updated the reverted fix for critical bug EXT-1655 ([BSI] Always flying after pressing "Stand" to stand up from an object), also integrated the fix for EXT-6786 ('Stand' button is corrupted if movement control floater is opened). Added some clarifying comments.
Reviewed by Tofu Linden at https://codereview.productengine.com/secondlife/r/253/.

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branch : product-engine