[Exim] $home in filter when check_local_user is not set

I'm using exim-4.04 in an unusual config; I'm not using
check_local_user but I'm setting user = based on something
else than the $local_part.

This all works fine, I use $address_data to pass things like
username and homedir around.

However when I execute a filter, $home is not set, and I
would like it to be (for logging, for example).

I supposed that transport_current_directory and
transport_home_directory would also influence the settings
for filtering but they don't.

Now I see two solutions:

- Add an option called 'local_user' that makes exim look at
the value of that option instead of $local_user when
check_local_user is set

- Set $home to the value of transport_home_directory
even for filters

Both fixes are easy enough, in fact I already implemented
the first one for fun.

Philip, is this something that should be fixed, if so,
which solution do you prefer and would you like a patch?

Mike.