rl_helper: implement basic navigation keys
rl_helper: encage cursor to use one line only

It's easier to handle one line for input rather than multiple lines, but if
we want to read more bytes, some precautions are needed.
So we handle something like viewport, which will be bytes showed limited
by terminal columns size (here fixed to 80 chars).
rl_helper: add delete key behavior

When delete key is pressed, everything from right position is shifted to
left.
rl_helper: change feed char to insert behavior

Everytime a printable char is fed, the behavior is to insert it at
position (pos variable). So any chars from right needed to be shifted
one position to right.