Linux implementation using ffmpeg

It's difficult to support both old and new versions, so raise the
requirement to the oldest that is commonly used, which is CMake 3.10.2
on Ubuntu 18.04.

Or rpmbuild gets grumpy.

Add missing build requirements: libpng-devel, zlib-devel

Remove obsolete build requirements: libXfont-devel

Fix unexpected rpm macro expansion `%{VERSION}` which is supposed
to be passed through as literal to `rpm -q --qf`. This hasn't
caused problems before, but it was found to cause problems when
built from EL 8 when it expanded to "1.10.1".

Update RPM package build process to automatically detect and apply
the correct X.org patch.

This makes our builds directly compatible with most distributions
without packagers/users having to specify extra flags.

Running as a service on a SELinux system requires rules so we can
transition to our own context. We also need the proper permissions
to start new user sessions.

This sets up a more correct session as there are key tasks that
need to be performed by PAM. E.g. systemd will allocate cgroups
and start base services.

In order to easily handle this as a system service the mapping of
displays is now done via a configuration file.

It is the most common init system these days so it should not be
hidden in the contrib/ directory.

This also removes all old SysV files from the contrib packages.

Install some example files to make things more easily discoverable.

Remove the unused Java self sign variable, and make sure we don't
hard code a version as it needs to be adjusted before a build.

It is needed by x0vncserver.

They include a new enough version, so stop doing our own build.

Applets don't work anymore so remove everything that has to do
with serving them.

See issue #534

OpenJDK 7u111 on el6 now throws an exception if the TSA URL scheme
is https.

These files are not Red Hat or Ubuntu specific, so install them
on all Unix systems where the viewer is installed.

Obsoleted by 7461517

Obsoleted by 7461517