shlib.sh: provide overrideable bang_exit function

shlib.sh: provide overrideable bang_exit function

When `bang_setup` is called, it sets a trap on EXIT.
This will step on any previously set trap on EXIT.

Make the EXIT trap set by `bang_setup` call a new
`bang_exit` function as its last action.

Provide a placeholder `bang_exit` function that
does nothing.

This way the `bang_exit` function can be replaced
before calling `bang_setup` thereby avoiding loss
of any needed EXIT trap activity as a result of
the EXIT trap set by `bang_setup`.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>