startup: treat stdin as text instead of commands

[justinmk]

Motivation

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

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Concepts

Treat stdin as input by default (so the "-" file is not needed):

echo foo | nvim

It works even if file args are given:

echo foo | nvim file1.txt file2.txt

It also works with -Es (but not -es), useful for scripts:

echo foo | nvim -Es +"%print" +"q!"

When you want to execute Normal-mode commands, use -s - instead:

echo ifoo | nvim -s - 

Other alternatives for executing commands:

• 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

Todo/Notes

• HACK: main.c: -Es sets p_ut to avoid delay on the final inbuf_poll(p_ut) call

[justinmk]

I just remembered (happily) that @ZyX-I already worked on the -s - feature in #6299 . @ZyX-I is that still WIP? What's needed to get it merged?

[ZyX-I]

AFAIR only AppVeyor hang.