Add toggle for pivoting still players with the camera

See it in action: https://cdn.discordapp.com/attachments/705529445313806477/705529901402554388/srb20812.gif

IMPORTANT: Uses up the last pflag. I'm not actually sure this feature is deserving of it, but Chrispy and I really want it and it seems others in the community feel strongly too. Internally we discussed moving the control-related pflags to a separate player struct variable, but doing so for a patch update would break backwards compatibility with addons. If there are strong oppositions to using up this pflag or alternative suggestions as to how to avoid using it up, please inform!! :)