Nvim notes:
- Nvim does not support "-u DEFAULTS", that change is omitted.
- Also add 'shadafile' as an alias to 'viminfofile'.
- Deprecate 'viminfofile'.

Problem:    Not easy to start Vim cleanly without changing the viminfo file.
            Not possible to know whether the -i command line flag was used.
Solution:   Add the --clean command line argument.  Add the 'viminfofile'
            option.  Add "-u DEFAULTS".
https://github.com/vim/vim/commit/c4da113ef98dcfd6f2a088b1693c0317dcb5bf42

    $ LC_ALL=en_US.UTF-8 MANROFFSEQ='' MANWIDTH=80 man --warnings -E UTF-8 -l -Tutf8 -Z man/nvim.1 >/dev/null
    mdoc warning: A .Bl directive has no matching .El (#59)

- develop.txt is for design/guidelines; architecture/concepts should
  live elsewhere (currently src/nvim/README.md)
- move dev-jargon to intro.txt
- replace https://neovim.io/community (deprecated) with
  https://neovim.io/#chat
- <Cmd> avoids CmdlineEnter/Leave
  https://github.com/vim/vim/issues/2889

roff/mdoc reference:
http://man7.org/linux/man-pages/man7/groff_mdoc.7.html

This changes Ex mode (Q, -e) to work like Vim's "improved Ex mode"
(gQ, -E).  That brings some small behavior differences, but should not
impact most Ex scripts (unless, for example, they depend on mappings
being disabled--but that can be solved for -e by skipping user config).

Before this change:
* the screen test hangs.

After this change:
* Q acts like gQ.
* -e/-es differs from -E/-Es only in its treatment of stdin.

This moves towards potentially removing getexmodeline().
(HINT: That does NOT mean "removing Ex mode", it means removing the
Vi-compatible Ex mode, which differs from Vim's "improved Ex mode" only
in some minor details (e.g. mappings are disabled).)

ref #1089 :-)~

closes #8043

doc: termios defaults. ref #6992
doc: :help shell-powershell
doc: provider: Python minimum version is 2.7, 3.4
doc: remove :!start special-case. #5844
doc: mention #7917 change which accepts empty Array for Dictionary parameter
doc: <Cmd> pseudokey
doc: lmap change #5658
doc: -s, -es

Fixes 2 failing tests in startup_spec.lua.

The Windows-only `--literal` option complicates support of "stdin-as-text
+ file-args" (#7679).  Could work around it, but it's not worth
the trouble:
- users have a reasonable (and englightening) alternative: nvim +"n *"
- "always literal" is more consistent/predictable
- avoids platform-specific special-case

Unrelated changes:
- Replace fileno(stdxx) with STDXX_FILENO for consistency (not motivated
  by any observed technical reason).