Provide a layer of indirection between the readline compatibility functions
and our internal implementation, so that we have the freedom to change the
function signature.
The place to change the completion_append_character is
usually somewhere in the `rl_completion_entry_function'
callback which is where one usually can distinguish between
file- or dir-like entries to append a slash for dirs etc.
This does no longer work since `fn_complete()' takes the
`append_character' as argument before the callback is executed,
so that changes to the variable `rl_completion_append_character'
have in fact no effect for the current completion.
Fix by adding a function that returns the rl_completion_append_character,
when it gets passed in a filename in readline emulation.
Pass in loads of parameters instead of relying on shed-loads of global
variables to modify the behaviour.
The filename completion code can now be enabled by code that uses el_gets().
(eg /bin/sh)
