SSL and USER_CERT_FILE round 2

Adding "sslkey" and "sslcert" to the PQconnectdb connection string.

After some discussion, I think it is more appropriate to add the key/cert
file for SSL into the connect string. For example:

PQconnectdb("host=foo dbname=bar sslmode=require
sslkey=/opt/myapp/share/keys/client.key
sslcert=/opt/myapp/share/keys/client.crt");


Any comments?