bashrc: PS1 calculation doesn't require other defined functions

Doing a `sudo su` was painful before because `face0` and `face1` weren't
defined in the root user's space.
Did a cleanup of how PS1 is calculated.
PS1 now leaves the prompt on a new line that mimics a new PS2.
bashrc: stbt-sink function to turn sink on/off without manual editing

Just a convenience function.