crtools: Introduce the --ext-mount-map option (v3)

On dump one uses one or more --ext-mount-map option with A:B arguments.
A denotes a mountpoint (as seen from the target mount namespace) criu
dumps and B is the string that will be written into the image file
instead of the mountpoint's root.

On restore one uses the same --ext-mount-map option(s) with similar
A:B arguments, but this time criu treats A as string from the image's
root field (foobar in the example above) and B as the path in criu's
mount namespace the should be bind mounted into the mountpoint.

v3:
* Added documentation
* Added RPC bits
* Changed option name into --ext-mount-map
* Use colon as key and value separator

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>