DWIM: avoid empty buffer 1 when stdin was empty. If other files were
specified at startup, we assume that stdin is only accidentally
not-a-TTY: user did not intend to send text from it.

ref #8560
ref #8561

If stdin is not a TTY we read it into buffer 1, as text. But if the
stdin pipe is empty, Nvim was most likely invoked for some other reason.
DWIM: select buffer 2 (if it exists). Example:

    echo file1 | xargs nvim

closes #8560
closes #8561
ref https://github.com/equalsraf/neovim-qt/issues/417

Before this change, -E/-Es without `-u NONE` reads stdin as Ex commands.
It should always read stdin as text (into buffer 1), like this:

    echo foo | nvim -Es +'%p'
    foo
    echo foo | nvim -Es -u NORC +'%p'
    foo

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 :-)~

This behavior was changed (improved) by 5861dc596687 which actually
makes -E more faithful to Vim's behavior.

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).

silent-mode (-es/-Es) has been broken for years.  The workaround up to
now was to include --headless.  But --headless is not equivalent because
it prints all messages, not the limited subset defined by silent-mode.

Treat stdin as text by default (so the "-" file is not needed):
    echo foo | nvim

It works with file args (implemented in next commit), too:
    echo foo | nvim file1.txt file2.txt

Why? Because:
- Execution of input is (1) almost always unintentional/confusing,
  and (2) potentially destructive.
- Avoids the need for time-delayed warning.  #7659
- The _common_ case is to open text in a buffer, not send commands.

Note:
- Not for Ex-mode (-es) because it is used by scripts.  But maybe `-Es`?
- Not for --headless, because stdio may be a protocol stream and may be
  used for any purpose by stdioopen().

To treat stdin as Normal-mode commands, use `-s -` instead:
    echo ifoo | nvim -s -
Other alternatives:
  - Replay a register. E.g. the following mostly works, except @q aborts
    on any "beep" (e.g. if the cursor can't move).
    nvim -c '%d q|norm @q' -
  - Future: Let `:%source` work with unsaved buffer contents?

closes #2087
closes #7659