[PATCH] shell: Add a ping command.

[Chris Johns]

This patch adds a ping command to the RTEMS shell. It pings for a count
of 5 or so and then stops because there is not ^C in the RTEMS shell.
A flood ping also works with the -c option. As stated in the commit
message some options work and some do not and I suspect the age of our
TCP/IP stack is the reason.

I have added doco to the patch but I have not built the doco because of
the hassle on MacOS.

This code needs to be changed to not use select and so avoid the issue
of more than 64 fd's open.