Add Linux-specific recvmmsg(2) and sendmmsg(2)

There is a compatibility issue regarding the type of a flag parameter:
int vs uint. Linux does declare the syscall to use uint and musl
followed it, but it is incompatible with other POSIX recv*/send*
syscalls. So it seems to be wise to follow the glibc version of
prototypes with int.

Signed-off-by: NODA, Kai <nodakai@gmail.com>

libc-test/build.rs

@@ -404,6 +404,11 @@ fn main() {
             "prlimit" | "prlimit64" |        // non-int in 2nd arg
             "strerror_r" if linux => true,   // actually xpg-something-or-other
 
+            // int vs uint. Sorry musl, your prototype declarations are "correct" in the sense that
+            // they match the interface defined by Linux verbatim, but they conflict with other
+            // send*/recv* syscalls
+            "sendmmsg" | "recvmmsg" if musl => true,
+
             // typed 2nd arg on linux and android
             "gettimeofday" if linux || android || freebsd || openbsd || dragonfly => true,
 

src/unix/notbsd/linux/mod.rs

@@ -220,6 +220,11 @@ s! {
         pub msgtql: ::c_int,
         pub msgseg: ::c_ushort,
     }
+
+    pub struct mmsghdr {
+        pub msg_hdr: ::msghdr,
+        pub msg_len: ::c_uint,
+    }
 }
 
 pub const ABDAY_1: ::nl_item = 0x20000;
@@ -983,6 +988,13 @@ extern {
                    -> ::ssize_t;
 }
 
+extern {
+    pub fn sendmmsg(sockfd: ::c_int, msgvec: *mut mmsghdr, vlen: ::c_uint,
+                    flags: ::c_int) -> ::c_int;
+    pub fn recvmmsg(sockfd: ::c_int, msgvec: *mut mmsghdr, vlen: ::c_uint,
+                    flags: ::c_int, timeout: *mut ::timespec) -> ::c_int;
+}
+
 cfg_if! {
     if #[cfg(any(target_env = "musl",
                  target_os = "fuchsia",