Add a menu item which functions similar to acme's `Look` command. This is copied from 9front. See: https://code.9front.org/hg/plan9front/rev/1f1596dbca51 https://code.9front.org/hg/plan9front/rev/d2de1d2f7b48

Change-Id: Id99ce9a143a21009d93a1c6ba2d1f84a3588649f Reviewed-on: https://plan9port-review.googlesource.com/2942 Reviewed-by: Russ Cox <rsc@swtch.com>

Change-Id: Id441d4df192c47388af6b5da306f14d90f066d18 Reviewed-on: https://plan9port-review.googlesource.com/1173 Reviewed-by: Russ Cox <rsc@swtch.com>

Ignore scroll/noscroll window setting. Instead, scroll when the write begins in or immediately after the displayed window content. In the new scrolling discipline, executing "Noscroll" is replaced by typing Page Up or using the mouse to scroll higher in the buffer, and executing "Scroll" is replaced by typing End or using the mouse to scroll to the bottom of the buffer. R=r, r2 http://codereview.appspot.com/4433060

R=rsc CC=codebot http://codereview.appspot.com/1044043