Early WIP: it kinda works.

Goal of the PR is to be able to set the signcolulmn width (currently stuck with 2 cells).
Follow up of #5802.

It adds a variable signcolumnwidth that indicates the signcolumn width when signcolumn==yes and the maximum width when signcolumn=auto.

So to test, I usually run LC_ALL=C make && ./build/bin/nvim -S ~/test_signs.vim ~/zaza:
with test_signs.vim:

:sign define a text=a
:sign define z text=z
:sign place 22 line=1 name=neomake_err buffer=1
:sign place 23 line=1 name=a buffer=1
:sign place 24 line=1 name=neomake_info buffer=1
:sign place 25 line=1 name=z buffer=1
:sign place 26 line=1 name=neomake_warn buffer=1

and then sets

set signcolumn=auto
set signcolumnwidth=6