Try to detect user/database/host from connection strings if

Try to detect user/database/host from connection strings if
log_connection is enabled and log_line_prefix doesn't include
them.

Extend the regex to autodetect database name, user name, client
ip address and application name. The regex now are the foolowing:

	db => qr/(?:db|database)=([^,]*)/;
	user => qr/(?:user|usr)=([^,]*)/;
	client => qr/(?:client|remote|ip|host)=([^,]*)/;
	appname => qr/(?:app|application)=([^,]*)/;

I hope this patch will limit the number of RTFM support answer.