(uce-reply-to-uce): Do not assume all Received lines
are on top of message without headers like `From' or `To'.
(uce-reply-to-uce): Parse Received lines better.
(uce-mail-reader): New user option.
(uce-reply-to uce): Add support for Gnus. User is supposed to set
uce-mail-reader to `gnus' if using Gnus to read mail. The default is
to assume Rmail. There's no magic to determine what mail reader is
currently active, so it is not possible to mix using uce.el with Rmail
and Gnus.
