tcp: options clean up

This should fix the following bugs:
  * Connections with MD5 signatures produce invalid packets whenever SACK
    options are included
  * MD5 signatures are counted twice in the MSS calculations

Behaviour changes:
  * A SYN with MD5 + SACK + TS elicits a SYNACK with MD5 + SACK

    This is because we can't fit any SACK blocks in a packet with MD5 + TS
    options. There was discussion about disabling SACK rather than TS in
    order to fit in better with old, buggy kernels, but that was deemed to
    be unnecessary.

  * SYNs with MD5 don't include a TS option

    See above.

Additionally, it removes a bunch of duplicated logic for calculating options,
which should help avoid these sort of issues in the future.

Signed-off-by: Adam Langley <agl@imperialviolet.org>
Signed-off-by: David S. Miller <davem@davemloft.net>