Introduce and use internal/linux

This package is to provide unix.* wrappers to ensure that:
 - they retry on EINTR;
 - a "rich" error is returned on failure.

 A first such wrapper, Sendmsg, is introduced.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

internal/linux/linux.go

@@ -0,0 +1,15 @@
+package linux
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// Sendmsg wraps [unix.Sendmsg].
+func Sendmsg(fd int, p, oob []byte, to unix.Sockaddr, flags int) error {
+	err := retryOnEINTR(func() error {
+		return unix.Sendmsg(fd, p, oob, to, flags)
+	})
+	return os.NewSyscallError("sendmsg", err)
+}