Support sslrootcert=system in the connection string

[contrun]

There is a special value for sslrootcert system introduced in postgresql 16.

https://www.postgresql.org/docs/17/libpq-connect.html

The special value system may be specified instead, in which case the system's trusted CA roots will be loaded. The exact locations of these root certificates differ by SSL implementation and platform. For OpenSSL in particular, the locations may be further modified by the SSL_CERT_DIR and SSL_CERT_FILE environment variables.

This will make ssl connection to the postgresql server much less painful.