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.

The function g_get_real_name is used to get user's full name, but when the full
name is not existed, it will return an empty string instead of NULL.

Bug fix: screensaver and lock screen not correctly displayed in configurations with overlapping monitor geometries

Switch to GdkSeat for GTK+ 3.20 and newer

This changes the code quite a lot as both grabbing the pointer and the keyboard
have to be done simultaneously now.

and set grid property only once, after grid is created

Change shake times to even number, so the dialog can return to its original position.

Avoid deprecated gdk_window_set_background functions

Which are completely broken in GTK+ 3.22.

Update mate-screensaver NEWS to use consistent, project wide, markdown-like formatting. This will make generating release announcements easier.

and require libmatekbd and libmate-desktop >= 1.17