* Password dialog is now scaled
* Background of lock screen is now scaled

ignoring return value of ‘fscanf’, declared with attribute warn_unused_result

This commit reverts:

https://github.com/mate-desktop/mate-screensaver/commit/816394c1a6ce9968dba3e1b0ecc884c8ccca4d43

And it applies an alternative to fix the deprecated functions:

gdk_screen_get_width
gdk_screen_get_height

Fixes #101

OpenBSD does not have nor use pam(8) for password authentication but
instead uses bsd_auth(3): add a bsd_auth authentication scheme to
make locking possible.
This has been a part of gnome-screensaver already but was probably
removed at one point. This is mostly identical to that code, with some
minor modifications.