cookie: cleanups and improvements

cookie: cleanups and improvements

- Stricter cookie validation with earlier rejection of empty/invalid
  cookie names

- secure and httponly attributes no longer accept = with empty values
  (only bare keywords)

- Validation checks (length, TAB, prefixes) moved into the first
  name/value pair block for better code organization

- Deferred time(NULL) calls for better performance when expires/max-age
  aren't used

- Simplified loop control flow by removing done flag

- The cookie size restriction now only applies to name + value, not other
  parts of the header line.

- Fixed a gcc 4.8.1 quirk

Closes #19868