support IPPROTO_MPTCP

Simply add the support for MPTCP protocol by adding
and exposing the MPTCP const Protocol.

MPTCP allow end user to open multiple path (TCP) to
the destination server using multiple interface. For
example, one can use the LTE connection of his phone
as well as a Wifi connection. Or Ethernet and Wifi.

Setting this Protocol will fail if the host does not
support MPTCP, thus the fallback is to open as TCP
like one would normally do.

For reference:
`sysctl net.mptcp.enabled=1`

Signed-off-by: Martin Andre <martin.andre@tessares.net>

src/lib.rs

@@ -302,6 +302,10 @@ impl Protocol {
 
     /// Protocol corresponding to `UDP`.
     pub const UDP: Protocol = Protocol(sys::IPPROTO_UDP);
+
+    #[cfg(target_os = "linux")]
+    /// Protocol corresponding to `MPTCP`.
+    pub const MPTCP: Protocol = Protocol(sys::IPPROTO_MPTCP);
 }
 
 impl From<c_int> for Protocol {

src/sys/unix.rs

@@ -63,6 +63,8 @@ pub(crate) use libc::SOCK_RAW;
 pub(crate) use libc::SOCK_SEQPACKET;
 pub(crate) use libc::{SOCK_DGRAM, SOCK_STREAM};
 // Used in `Protocol`.
+#[cfg(target_os = "linux")]
+pub(crate) use libc::IPPROTO_MPTCP;
 pub(crate) use libc::{IPPROTO_ICMP, IPPROTO_ICMPV6, IPPROTO_TCP, IPPROTO_UDP};
 // Used in `SockAddr`.
 pub(crate) use libc::{
@@ -386,6 +388,8 @@ impl_debug!(
     libc::IPPROTO_ICMPV6,
     libc::IPPROTO_TCP,
     libc::IPPROTO_UDP,
+    #[cfg(target_os = "linux")]
+    libc::IPPROTO_MPTCP,
 );
 
 /// Unix-only API.

tests/socket.rs

@@ -106,6 +106,8 @@ fn protocol_fmt_debug() {
         (Protocol::ICMPV6, "IPPROTO_ICMPV6"),
         (Protocol::TCP, "IPPROTO_TCP"),
         (Protocol::UDP, "IPPROTO_UDP"),
+        #[cfg(target_os = "linux")]
+        (Protocol::MPTCP, "IPPROTO_MPTCP"),
         (500.into(), "500"),
     ];