Close extraneous file descriptors in containers

Without this patch, containers inherit the open file descriptors of the daemon, so my "exec 42>&2" allows us to "echo >&42 some nasty error with some bad advice" directly into the daemon log. :)

Also, "hack/dind" was already doing this due to issues caused by the inheritance, so I'm removing that hack too since this patch obsoletes it by generalizing it for all containers.

Docker-DCO-1.1-Signed-off-by: Andrew Page <admwiggin@gmail.com> (github: tianon)

system/fds_unsupported.go

@@ -0,0 +1,12 @@
+// +build !linux
+
+package system
+
+import (
+	"fmt"
+	"runtime"
+)
+
+func CloseFdsFrom(minFd int) error {
+	return fmt.Errorf("CloseFdsFrom is unsupported on this platform (%s/%s)", runtime.GOOS, runtime.GOARCH)
+}