prompt: only use unicode chars on some terminals

For example, exclude "linux" as a multibyte enabled terminal
battery: use _vbe_can_do_unicode() function
locale: remove multibyte check

It seems it is bogus. Let ZSH handle this option.
Run zsh inside tmux
zshrc: use TMUX variable to check if we are running inside tmux
prompt: add back unicode check based on character length of an unicode char

It seems that multibyte will be true even if we are using a locale
that does not support unicode (like C).