Clarification of the format of control files, Closes: #501930, #593909.

Clarification of the format of control files, Closes: #501930, #593909.

 - Specifies field names similarly to RFC 822/5832;
 - Distinguishes simple, folded and mulitiline fields;
 - Clarifies paragraph separators (#501930);
 - The order of paragraphs is significant;
 - Fields can have different types or purposes in different control files;
 - Moved the description of comments from §5.2 to §5.1;
 - Documented that relationship fields can only be folded in debian/control.