cvs co -d <repository> prevention

[Donald Sharp]

If the user has set his CVSROOT env variable and calls cvs in this
manner:

cvs co -d <repository> <module>

The current version will happily check the module out inside of 
the repository.

I've attached a patch that fixes this problem( It's not based on
the other checkout into the repository patch I got going at this
point in time. ).  I've also included a sanity.sh modification
that shows tests this case ;)

Please let me know if you have any suggestions/comments.

Thanks!

donald

checkout_repository_d_option
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