Rename some bash powerline functions

Renamed `_powerline_prompt` to `_powerline_set_prompt` and `_run_powerline` to 
`_powerline_prompt.`

Reason: name `_run_powerline` is not consistent with other names used in shell 
bindings.
Refactor _powerline_init_tmux_support

- Move _powerline_tmux* functions definitions out of
  _powerline_init_tmux_support.
- Do not touch PROMPT_COMMAND unless tmux support is to be added.