BUG/MINOR: acl: implicit arguments of ACL keywords were not properly resolved

William Lallemand reported a bug which happens when an ACL keyword using an
implicit argument (eg: a proxy name) is used : the keyword is not properly
set in the arglist field, resulting in an error about the previous keyword
being returned, or "(null)" if the faulty ACL appears first.

The bug only affects error reporting and is 1.5-specific, so no backport is
nedeed.