Suggested by G. Brandon Robinson.

The overloading of IR emits magic \X'...' sequences that turn into HTML manual links. But not all such IR invocations should be manual links; those had to be written to avoid the IR macro before. Worse, the \X'...' ending the IR causes troff to emit only a single space after a period. Defining a new IM macro for manual references fixes both problems. Fixes #441.

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