Make termguicolor use gui= instead of cterm=

[whydoubt]

The behaviour for a color terminal has been:
termguicolor: use guifg, guibg, cterm
notermguicolor: use ctermfg, ctermbg, cterm

This is changed to the more consistent:
termguicolor: use guifg, guibg, gui
notermguicolor: use ctermfg, ctermbg, cterm

Fixes issue GH-1740

[codecov-io]

Codecov Report

Merging #1773 into master will decrease coverage by 0.01%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master    #1773      +/-   ##
==========================================
- Coverage   75.11%   75.09%   -0.02%     
==========================================
  Files          76       76              
  Lines      124994   125006      +12     
==========================================
- Hits        93886    93878       -8     
- Misses      31108    31128      +20

Impacted Files	Coverage Δ
src/screen.c	74.31% <50%> (-0.03%)	⬇️
src/syntax.c	77% <87.5%> (-0.11%)	⬇️
src/if_xcmdsrv.c	84.25% <0%> (-1.12%)	⬇️
src/message.c	68.07% <0%> (-0.41%)	⬇️
src/os_unix.c	58.63% <0%> (-0.2%)	⬇️
src/gui.c	45.58% <0%> (-0.06%)	⬇️
src/channel.c	83.8% <0%> (ø)	⬆️
src/window.c	81.38% <0%> (+0.03%)	⬆️
src/term.c	53.37% <0%> (+0.05%)	⬆️
... and 2 more

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[brammool]

Jeff Smith wrote:

The behaviour for a color terminal has been: termguicolor: use guifg, guibg, cterm notermguicolor: use ctermfg, ctermbg, cterm This is changed to the more consistent: termguicolor: use guifg, guibg, gui notermguicolor: use ctermfg, ctermbg, cterm Fixes issue GH-1740

This makes a lot of sense. But it is not backwards compatible. Would any users be upset that we change this? And would that be enough to justify making this an option? I hope most users will be happy to get this change.

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[brammool]

I'm closing this, since there hasn't been enough request for the change to justify the backwards-incompatibility. It would be possible to add a new argument for attributes used both by the gui and cterm+termguicolors, but that's a bit complicated.