Check magic cookies between remote endpoints (#22)

The idea here is to ensure endpoints are properly paired by exchanging
a "cookie." If the source side cookie and destination side cookie
don't match then the transfer will fail immediately. Our inspiration
is XWindows and MIT-MAGIC-COOKIE-1.

This feature won't be useful until the cookie can be set by a command
line option.
Add a '-cookie' CLO to set the magic cookie (#22)

This commit fixes #22.
Merge pull request #40 from ORNL/fix-22

Fix #22