PostgreSQL dblink and subscriptions can append to arbitrary files

External control of file name in PostgreSQL dblink and CREATE/ALTER SUBSCRIPTION allows a user to achieve persistent denial of service, via appending invalid data to a chosen file. Specifically, the "sslkeylogfile" connection parameter appends data, in NSS keylog format, to a file of the user's choosing. This "sslkeylogfile" mechanism cannot add arbitrary bytes. Within major version 18, minor versions before PostgreSQL 18.7 are affected. Versions before PostgreSQL 18 are also affected if libpq version 18 or later is installed and in use by the server.