Return a coding system to encode the outgoing message of the current buffer.
It at first tries the first coding system found in these variables
in this order:
(1) local value of `buffer-file-coding-system'
(2) value of `sendmail-coding-system'
(3) value of `default-sendmail-coding-system'
(4) default value of `buffer-file-coding-system'
If the found coding system can't encode the current buffer,
or none of them are bound to a coding system,
it asks the user to select a proper coding system.