#19 $user_id_sql as config-option

[Kaleissin]

As of amavisd-new 2.3.3, $user_id_sql is hardcoded to
be 'id'. Since it is possible to change
$sql_select_white_black_list it must either
1) be possible to also change $user_id_sql or better
the necessary piece inside $user_id_sql.
2) be documented, clearly, that that the table of
recipients' field for ids can only be 'id'.

Believe it or not, but there ar places that ban
sql-fields carrying only the name 'id'. In such places
there always must be a way to attempt to show which
table the id belongs to, and what I have seen used is
prefixing 'id' with the first letter of the table-name,
or even prefixing the entire table-name. So one way or
another, in such places $user_id_sql needs to conform.
It is better to put it in the config though than
patching amavisd-new by hand everytime there's an
upgrade...