dot/shell-rc, dot/bashrc, dot/zshrc: Introduce a formal notion of hooks.

Replace the `__mdw_precmd' and `__mdw_preexec' functions with hooks.
Each shell implementation is responsible for calling the hooks at the
right time, but the common core is responsible for maintaining the hooks
and doing the job of invoking the hook functions.

Use this machinery for the prompt setting and `screen' status things.