Due to go std lib uses poller for os.File introducing in this commit:
https://github.com/golang/go/commit/c05b06a12d005f50e4776095a60d6bd9c2c91fac


There are two changes to watch out:
1. os.File.Fd will always return a blocking fd except on bsd.
2. os.File.Read won't return EAGAIN error for nonblocking fd.

So
For 1, we just get tty's fd in advance and then set its block mode.
For 2, we use read syscall directly to get what we wanted error(EAGAIN).

Fix issue #910.

Signed-off-by: Tw <tw19881113@gmail.com>

Close #906

Do not automatically decide to use alternate screen when the value of
height exceeds the height of the terminal.

    # Use alternate screen
    fzf
    fzf --height 100%
    fzf --no-height

    # Still use current screen
    fzf --height 10000

Fix broken preview border. Reported by Thomas Sattler.

    fzf --bind 'enter:execute(date)' --preview=date --reverse

Goes well with --height

- Move cursor to the top-left corner when starting a command in
  alternate screen
- Fix cursor position when returning to alternate screen when fzf is
  running in full screen mode

Close #825

- Remove unnecessary scrolling
- Allow us to use `--height 100%` under Neovim terminal for 24-bit colors

Related:
- #789
- https://github.com/neovim/neovim/issues/4151

Also revert the workaround in Vim plugin introduced in fa7c8977.

Related: #814

Close #798

Excerpt from http://www.tldp.org/HOWTO/Bash-Prompt-HOWTO/x361.html:

> - Save cursor position:
>   \033[s
> - Restore cursor position:
>   \033[u
>
> The latter two codes are NOT honoured by many terminal emulators. The
> only ones that I'm aware of that do are xterm and nxterm - even though
> the majority of terminal emulators are based on xterm code. As far as
> I can tell, rxvt, kvt, xiterm, and Eterm do not support them. They are
> supported on the console.

They are also unsupported by Neovim terminal.