xapian: look for starting list-id angle bracket from end of string

Some mail services use HTML tags in the phrase part of RFC2919
List-Id header values (and tend to forget the closing angle
bracket on the actual list-id). Treating the last '<' character
as starting angle bracket allows us to propertly extract the
list-id value from such headers.

This also works nicely with variants where clients cannot make
up their mind if their list-ids should be RFC2919 or Groups-style.