Extend the capabilities of is_os to detect more platforms such as
freebsd and openbsd. Also remove `iswin()` helper function as it can be
replaced by `is_os("win")`.

The old behaviour (e.g. via `set display-=msgsep`) will not be available.
Assuming that messages always are being drawn on msg_grid
(or not drawn at all, and forwarded to `ext_messages` enabled UI)
will allows some simplifcations and enhancements moving forward.

Avoids using `gui=reverse` on `VertSplit` and makes window separators
look much nicer by default.

Problem:    The Normal highlight is not defined when compiled with GUI.
Solution:   Always define Normal. (Christian Brabandt, closes vim/vim#4072)
https://github.com/vim/vim/commit/f90b6e03a983b62b66564fc449e32724d6456769

* mode_spec: retry with increasing matchtime

`matchtime=2` might still be too low (with luacov on AppVeyor).

    [  ERROR   ] test/functional\ui\mode_spec.lua @ 47: ui mode_change event works in insert mode
    test\functional\ui\screen.lua:587: mode
    Expected objects to be the same.
    Passed in:
    (string) 'insert'
    Expected:
    (string) 'showmatch'
    Hint: full state of "mode":
      "insert"

Followup to fe60013fb.
ref #10941
Initially regressed in 7ed212262242

``

* ui/screen_basic_spec: set timeoutlen=10000

This fixes the test on slow CI.

Ref: https://ci.appveyor.com/project/neovim/neovim/builds/27600387/job/t468h2b3w9lwtlm5#L10930

But add an escape hatch needed for external TUI, so it still can use
terminal emulator defaults.