(rmail-retry-failure): Copy the whole block of headers from the message

(rmail-retry-failure): Copy the whole block of headers from the message
and then discard those in rmail-retry-ignored-headers.  Delete
usage of rmail-retry-setup-hook.  Bind mail-signature and
mail-setup-hook to nil when composing retry buffer.
Handle mail-self-blind.

(rmail-retry-ignored-headers): New variable,
specifying the headers that should be removed by rmail-retry-failure.
(rmail-retry-setup-hook): Obsolete variable (see below), deleted.
(rmail-clear-headers): New optional arg is list of headers to clear.