patch - ping select interface.

[Bob Keyes]

I originally made this for my own needs, contributed it to openwrt but 
think it should go upstream to busybox. I am new here, not sure of the 
regular way I am supposed to make contributions, but whoever is in charge 
of accepting patches please take a look at 
https://dev.openwrt.org/ticket/2522 which will explain the rational for 
the feature and includes the patch for it.

Regards,
Bob Keyes