Screensaver command-line arguments can now be configured via a gsettings
dictionary. It's also compatible with screensavers from XScreenSaver.

A helper script is included:
  mate-screensaver-configure starfield --count 500 --speed 10

Fixes #5

By the term "prompt-driven" I mean two-way conversation between the
screensaver dialog and the PAM stack. As you probably know, PAM works
by conversation with a dialog program asking the user to enter
something in answer to each message it sends. In the most conventional
case, the only question is "Password:" and the password is the only
data the user enters. But in general, the number of questions and
messages are not limited to that.

The previous support of PAM helper (gs-auth-helper.c) was written and
worked for the mentioned "only password" authentication scheme. For
other schemes it wasn't enough. New implementation fixes that
limitation.

Same as the previous version of gs-auth-helper.c, the new version uses
pipe interface for interprocess communication and synchronization.
However, unlike the previous version, new version uses two pipes
instead of a single pipe: the first one is used to transfer
prompt text from PAM via the helper to the screensaver dialog, and the
second one is used to transfer the user input from the dialog to
helper (and then the helper replies with it back to PAM). Having that
bidirectional prompt/reply channel it is possible to make as many
prompt/reply passes as required by PAM.

The present helper program (see the helper/ dir) is based on the
helper written by okir@suse.de, which is in turn loosely based on
unix_chkpwd by Andrew Morgan.

All new code is untabified under the assumption the tab width is 8.

Signed-off-by: Paul Wolneykien <manowar@altlinux.org>

Reuses WITH_SYSTEMD #define to reduce #ifdef mess.

ELOGIND is not officially supported, but since it's
a subset of systemd, it should work out of the box.

Co-authored-by: Joakim Tjernlund <joakim.tjernlund@infinera.com>

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.