more: refactor and clarify code

more: refactor and clarify code

Use the backspace provided dy terminfo, and if that is not possible use
fallback.

Simplify entering and exiting standard mode.  Now when these std_enter and
std_exit variables are initialized correctly they do not need to be checked
quite as complex way as earlier.

Do not call prepare_line_buffer() unnecessarily.  The line buffer needs only
to be prepared only at times of more(1) initialization and when terminal
size changes.

Add a function to print various separators, that is better than static
strings.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>