Add MPTCP's MP_CAPABLE option support

usage: mpcapable{[skey=<number>,rkey=<number>,csum=true|false]}
  - skey specifies the Sender's key (random if not set);
  - rkey specifies the Receiver's key (unused if not set);
  - csum specifies if DSS checksum is enabled (true if not set).

Constant value MPCAPABLE is similar as mpcapable{}.

Note that some fields are fixed:
  - Subtype is set to 0;
  - Version is set to 0;
  - bit H is set to 1;
  - bit B through G are set to 0.

Signed-off-by: Gregory Detal <gregory.detal@uclouvain.be>