restore behaviour of /usr/bin/clear command

[Dieken]

clear is meant to clear screen, not clear display which clear scrollback buffer too.

And unfortunately fish shell binds Ctrl-L to clear command, so Ctrl-L always clear
scrollback buffer if emacs-vterm.fish is sourced, this is very surprising.

Further more, both original clear command and Ctrl-L don't clear scrollback buffer
in iTerm2 and Terminal.app.

References:

1. man clear shows "clear the terminal screen".

2. In bash, bind -p | grep C-l shows:

    "\e\C-l": clear-display
    "\C-l": clear-screen

3. In zsh, bindkey | fgrep '^L' shows:

    "^L" clear-screen
    "^[^L" clear-screen

4. In fish, bind -a | fgrep '\cl' shows:

    bind --preset \cl echo\ -n\ \(clear\ \|\ string\ replace\ \\e\\\[3J\ \"\"\)\;\ commandline\ -f\ repaint

@jixiuf

[Dieken]

The behaviour of "clear" varies across different OS and terminfo database.