The abort came from using libuv tty handle on non-tty fd. Use uv_pipe_t in these
cases. Also add simple test for this case.

Refactor input.c, normal.c and edit.c to use the K_EVENT special key to trigger
the CURSORHOLD event. In normal and edit mode, K_EVENT is treated as
K_CURSORHOLD, which enables better handling of arbitrary actions in those
states(eg: In normal mode the previous operator counts will be restored).

Also fix a test in vim_spec.lua. The test had a wrong assumption: cmdheight is
only used to determine when the press enter screen will be shown, not to limit
how many lines or control pagination.

Problem:    Moving the cursor in Insert mode starts new undo sequence.
Solution:   Add CTRL-G U to keep the undo sequence for the following
            cursor movement command. (Christian Brabandt)

https://github.com/vim/vim/commit/8b5f65a527c353b9942e362e719687c3a7592309

Closes #3492

XDG base directory specification support

Target: make all tests run with chmod -x ~/.config/nvim ~/.local/share/nvim.