Fixed several issues in the pager_output code.

Pager bled color due to a bad check in ch->set_pager_color.
Output repeated the last line before the page prompt.
The "N" command for the prompt did not work.
The "R" command would refresh the current page, but cut one line off
each time it was called.
The "B" command would go back, but not far enough, causing the wrong
lines to redisplay.

This all addresses Issue #11.