Raise exception Bad_option for unparseable or invalid options.
Changes per @avsm feedback, w/exception of catching `Failure`
Add try block for get_uint16 failure in MSS checks; separate check_mss to own function.
Wrap variable-length option parsing in a `try` per @avsm's advice.
Merge pull request #58 from avsm/master

quieten logging from tcp when handling ipv6
s/Failure/Invalid_argument/g
Do not expose the `Tcpip_checksum` module twice in the `oasis`

Lift it up to the toplevel `Tcpip` library, since it's just external
function declaration and no other values.
Merge pull request #60 from avsm/fix-checksum-modules

Do not expose the `Tcpip_checksum` module twice in the `oasis`
Merge branch 'fix-tcp-unmarshalling' of git://github.com/yomimono/mirage-tcpip into yomimono-fix-tcp-unmarshalling
Add `opam` file to permit easier local pinning, and fix Travis to use this.