Refactor database configuration.
Refactor database configuration.

- don't repeat config-db.php file path

- add @is_readable,
	we might not have the proper permissions.

- use 'require_once' instead of 'include',
	this file should only be included once and we
	should fail dramatically if we can't load it.

- better documentation.

Yubico/yubikey-val,ahojjati/yubikey-val,ahojjati/yubikey-val,ahojjati/yubikey-val,Yubico/yubikey-val,Yubico/yubikey-val