The `herbstclient` alias in the generated `autostart` made it impossible
to use bash functions.
This makes the `herbstclient` alias optional by adding an extra
`herbstclientAlias` option on the herbstluftwm configuration. The new
option defaults to `false` as to not confuse newcomers to the
herbstluftwm module, which is not a breaking change because it was only
an optimization.
