Let psconvert -N+kfill set alternate fade fill

[PaulWessel]

See #7593 for some background. movie allows two fill settings that are passed to psconvert when building the rasters:

1. With -G one can optionally fill the bounding box with specified colour before other things are painted
2. With -K (and -E) one can specify what the terminal fade colour should be [Default is black].

Unfortunately, psconvert only had -N+g to accept the background fill and the fade colour was passed from movie the same way via -N+g, resulting in two different +gcolor modifier passed to psconvert, which then of course used just the last one given. Thus, fade was always black.

This PR does several things:

1. Adds +kfadefill to -N and updates movie to pass that modifier to psconvert when the fading colour is explicitly set.
2. Deprecates -N+i for ancient ghostscript versions. We honour it under the hood but drop the documentation and the reasoning behind it.
3. Fix a few missing psconvertoptions in figure
4. Use gmt_rgb_syntax not gmt_pen_syntax to complain about bad rgb fill

Closes #7593

ffmpeg3.mp4

[Esteban82]

I see now that the default color for the fade is black while the canvas color is white (no fill). Wouldn't be better to have the same default color? I know that it is not important.

[PaulWessel]

If they both were white or black, you would fade from white to white or black to black. Since -G changes background from white it makes most sense to me to default fade from black if -K or -E is used.

[Esteban82]

If they both were white or black, you would fade from white to white or black to black.

Mmm, yes, you will see the content of the last frame dissapear (to white or black).

But, I am ok with your solution.