Refactor the "confirmation code" stuff into a general purpose script.

confirm-code.cgi contains a preconfigured list of databases and their
parameters. When a request comes in, the database in the request's 'db'
parameter is checked for validity, and a code is generated, stored in
the appropriate database and returned.

Use this new script in send-pr.sgml and remove sendpr-code.cgi which is
now superceded.