Deprecate and remove terminals linux and linux vgagl

[sfeam <sf...@us...>]

This proposed action is triggered by a recent support request for use of
gnuplot from the linux console command line (no X11 present).

Please contribute any comments or anecdotes about using gnuplot from the
linux console.  


Proposed action
===============

Version 5.2: Mark both terminals DEPRECATED
Version 5.3: Remove the configuration option --with-linux-vga
             Remove linux.trm and vgagl.trm from the current source

Background
==========

Both the "linux" and "vgagl" terminals rely on gnuplot being able to
write directly to the system graphics device (/dev/vga or /dev/svga or
/dev/console or various other synonyms).  This is accomplished by calling
into libraries libvga or libsvga (both are provided by the svgalib
source). The gnuplot executable must be installed suid root so that it
has sufficient privilege for this to work.  A corresponding linux
kernel module svgalib_helper must be available and loaded into the kernel.

This was reasonable 20 years ago; now, not so much.  

Nothing in a typical modern linux system requires or uses svgalib and
most distros do not include it by default.  It is not clear that it
is even possible to build it from existing source to work with current
linux kernels.  I have not been able to get it to work since gnuplot
version 4.2 ten years ago, although I have not really tried very hard. 

The gnuplot core routines supporting pm3d color had an alternative
code path controlled by #ifdef EXTENDED_COLOR_SPECS that affected all
terminals if the linux vgagl terminal was configured.  This code path
has not been maintained and no longer works in 5.3 so I removed it.
That means currently the vgagl terminal would not compile even if a
working svgalib were available. 


Replacement
===========

"set term dumb" is an option of course, but we can do better.

Various terminal interfaces can be run at the linux console level.
At least one of these (yaft) supports sixel graphics.
Therefore one way to run gnuplot from the linux console is to
run yaft (https://github.com/uobikiemukot/yaft) as your console
interface, run gnuplot from the yaft command line, and select
"set term sixel" for output.  I was able to do this successfully
on a laptop running linux kernel 4.17 and no special configuration
of either yaft or gnuplot. This seems an adequate replacement
for the original vgagl terminal, although neither of the current
gnuplot sixel output modes supports mousing.