fix message handling to use object to get command string returned by
find_first_plaintext_message().

public -> private:
o head_message
o last_message
o next_message
o prev_message

prefix change:
o rfc822_message_ -> whole_message_
o get_first_plaintext_message() -> find_first_plaintext_message()
o get_encoding_mechanism() -> encoding_mechanism()
o get_data_type() -> data_type()
o get_data_type_list() -> data_type_list()

renamed:
o get_offset() - >offset_pair()
o header_in_body_part() -> message_fields()
o data_in_body_part()   -> message_text()
where we use RFC822 flabour naming convention.
rfc822 says "message = fields + text" and rfc2047 says "message" as
defined in RFC 822, with all header fields optional.