loader(8): adjustment & man update

loader(8): adjustment & man update

 * fix `help' command by using $base/loader.help,
otherwise `help' didn't work when pwd outside $base.
 * allow `cd' and `optcd' without arguments,
using $base as default.
 * use pager for `?' and `local' commands.
 * make `more' work with relative file names
 * truncate very long lines (more than 256 chars),
and give warning; before they stopped include processing;
these are typically just comments.
 * add `optcd kernel' when using ESC in `menu',
like the menu item (`9') that exits via dloader.menu.
 * remove extra line in Fred menu (on right)
(it was scrolling warning about truncated line off screen).
 * remove obsolete variables in loader.conf.
 * update `help' output.
 * update man pages.