libct: add mountViaFDs, simplify mount

1. Simplify mount call by removing the procfd argument, and use the new
   mount() where procfd is not used. Now, the mount() arguments are the
   same as for unix.Mount.

2. Introduce a new mountViaFDs function, which is similar to the old
   mount(), except it can take procfd for both source and target.
   The new arguments are called srcFD and dstFD.

3. Modify the mount error to show both srcFD and dstFD so it's clear
   which one is used for which purpose. This fixes the issue of having
   a somewhat cryptic errors like this:

> mount /proc/self/fd/11:/sys/fs/cgroup/systemd (via /proc/self/fd/12), flags: 0x20502f: operation not permitted

  (in which fd 11 is actually the source, and fd 12 is the target).

   After this change, it looks like

> mount src=/proc/self/fd/11, dst=/sys/fs/cgroup/systemd, dstFD=/proc/self/fd/12, flags=0x20502f: operation not permitted

   so it's clear that 12 is a destination fd.

4. Fix the mountViaFDs callers to use dstFD (rather than procfd) for the
   variable name.

5. Use srcFD where mountFd is set.

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

libcontainer/console_linux.go

@@ -18,7 +18,7 @@ func mountConsole(slavePath string) error {
 	if f != nil {
 		f.Close()
 	}
-	return mount(slavePath, "/dev/console", "", "bind", unix.MS_BIND, "")
+	return mount(slavePath, "/dev/console", "bind", unix.MS_BIND, "")
 }
 
 // dupStdio opens the slavePath for the console and dups the fds to the current