dccp: Silently ignore options with nonsensical lengths

This updates the option-parsing code with regard to RFC 4340, 5.8:
 "[..] options with nonsensical lengths (length byte less than two or more
  than the remaining space in the options portion of the header) MUST be
  ignored, and any option space following an option with nonsensical length
  MUST likewise be ignored."

Hence in the following cases erratic options will be ignored:
 1. The type byte of a multi-byte option is the last byte of the header
    options (i.e. effective option length of 1).
 2. The value of the length byte is less than the minimum 2. This has been 
    changed from previously 3: although no multi-byte option with a length
    less than 3 yet exists (cf. table 3 in 5.8), a length of 2 is valid.
    (The switch-statement in dccp_parse has further per-option length checks.)
 3. The option length exceeds the length of the remaining option space.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>