linux: Add definitions for Multipath TCP

The Linux kernel recently gained support for [Multipath TCP](https://en.wikipedia.org/wiki/Multipath_TCP), a protocol which builds upon TCP. This patch adds `IPPROTO_MPTCP` to open new sockets and `TCP_ULP` to query the upper layer protocol of traditional TCP sockets (which is set to `mptcp` for all underlying connections)

Source: [tcp: Define IPPROTO_MPTCP](https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=faf391c3826cd29feae02078ca2022d2f912f7cc)

cc #1896