VPP-23 Re-work pager line collation

To better handle lines longer than the terminal width, re-work how
the pager collates lines. We still store each output line in a vector
or lines; additionally we index the start and end of displayable lines.

The index then becomes the data to work out what to display.

If the terminal is resized this index is rebuilt and the current page
redisplayed and an attempt made to keep the current top-of-screen line
in place.

Change-Id: Icc6f4f72dabad89682cd82be88adb03eadb7811d
Signed-off-by: Chris Luke <chrisy@flirble.org>