(mail-interactive): Doc fix.

(mail-interactive): Doc fix.
(mail-yank-ignored-headers): Add X-RMAIL- headers, bump :version.
(mail-setup): Use the function mail-signature.
(mail-to, mail-subject, mail-cc, mail-bcc, mail-reply-to)
(mail-mail-reply-to, mail-mail-followup-to): Doc fixes.
(mail-signature): Check the signature file is readable.
With the argument ATPOINT, really insert at point.
Handle the case when the variable mail-signature is an expression.