Add `openssl ca -revoke <certfile>' facility which revokes a certificate

specified in <certfile> by updating the entry in the index.txt file.
This way one no longer has to edit the index.txt file manually for
revoking a certificate. The -revoke option does the gory details now.

Submitted by: Massimiliano Pala <madwolf@openca.org>
Cleaned up and integrated by: Ralf S. Engelschall