Make -CAserial a type 's' option

The file name given to -CAserial might not exist yet.  The
-CAcreateserial option decides if this is ok or not.

Previous to this change, -CAserial was a type '<' option, and in that
case, the existence of the file given as argument is tested quite
early, and is a failure if it doesn't.  With the type 's' option, the
argument is just a string that the application can do whatever it
wants with.

Reviewed-by: Rich Salz <rsalz@openssl.org>