Ensure that selinux is disabled by default

This also includes some portability changes so that the package can be
imported with the top level runtime.
Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

pkg/system/calls_linux.go

@@ -143,3 +143,7 @@ func SetCloneFlags(cmd *exec.Cmd, flag uintptr) {
 	}
 	cmd.SysProcAttr.Cloneflags = flag
 }
+
+func Gettid() int {
+	return syscall.Gettid()
+}

pkg/system/unsupported.go

@@ -13,3 +13,7 @@ func SetCloneFlags(cmd *exec.Cmd, flag uintptr) {
 func UsetCloseOnExec(fd uintptr) error {
 	return ErrNotSupportedPlatform
 }
+
+func Gettid() int {
+	return 0
+}