init: use *os.File for passed file descriptors

While it doesn't make much of a practical difference, it seems far more
reasonable to use os.NewFile to wrap all of our passed file descriptors
to make sure they're tracked by the Go runtime and that we don't
double-close them.

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

libcontainer/utils/utils_unix.go

@@ -202,3 +202,12 @@ func ProcThreadSelf(subpath string) (string, ProcThreadSelfCloser) {
 	}
 	return threadSelf + subpath, runtime.UnlockOSThread
 }
+
+// ProcThreadSelfFd is small wrapper around ProcThreadSelf to make it easier to
+// create a /proc/thread-self handle for given file descriptor.
+//
+// It is basically equivalent to ProcThreadSelf(fmt.Sprintf("fd/%d", fd)), but
+// without using fmt.Sprintf to avoid unneeded overhead.
+func ProcThreadSelfFd(fd uintptr) (string, ProcThreadSelfCloser) {
+	return ProcThreadSelf("fd/" + strconv.FormatUint(uint64(fd), 10))
+}