Improved auto-tmux start up

Shells inside tmux weren't behaving correctly even after being launched
with atmux.  Now loading auto-tmux for all shells inside tmux (by
checking $TMUX).
tmux enhancements: cwd-sync and cwd-title

- cwd-title makes tmux use the name of current working dir as window
  title.
- Separated cwd-sync from auto-tmux, which syncs default-path with the
  current working directory.
prompt setup automatically only for login shells

To also load on interactive subshells, set prompt_subshells=true in
your ~/.bash_local or simply use -prompt-setup.
Shorthand for detecting current working dir change

This is becoming a common idiom in many plugins, so adding:
- bash_insert_chpwd
- bash_add_chpwd
- -chpwd-detect
- $CHPWD_COMMAND
Updated xtermtitle to use bash_add_chpwd
xtermtitle hid some internal names
xtermtitle resets Apple Terminal icon for ssh/mosh
some notes on using `trap ... DEBUG` for xtermtitle