Return the value of the header field whose type is FIELD-NAME.
If second arg LAST is non-nil, use the last field of type FIELD-NAME.
If third arg ALL is non-nil, concatenate all such fields with commas between.
If 4th arg LIST is non-nil, return a list of all such fields.
The buffer should be narrowed to just the header, else false
matches may be returned from the message body.
(fn FIELD-NAME &optional LAST ALL LIST)