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.

This version of Ubuntu is no longer fully supported so let's stop caring
about builds there.

This was removed from Ubunut's X.org packages years ago, so drop it from
our packing as well.

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.

These are EOL (or soon to be) and we want to start relying on things
present in current distributions.

Make the builds faster as we now run them often via Travis.

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.

libXft is needed to build FLTK for the RHEL 6 RPM.

The supporting scripts weren't really up to date, so remove them
and just require the packager to include FLTK in the source
tarball.

It is needed by x0vncserver.

It is needed to build Xvnc.

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

Packages for RHEL 5 and Ubuntu 12.04 are no longer maintained, so
remove those files.

Our main web site now supports https, so update all references.

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

See issue #534

Fixes issue #408

Fixes #360

Fixes #349

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