Change formspec to use textarea element.

This also removes the pipe as line separator, and uses real
newlines instead.

To be compatible with older signs, those older signs are *NOT*
updated, and still use the old line handling code. In order to
take advantage of the new UI, old signs will have to be
replaced. This is an unfortunate drawback that cannot be
fixed.

Newly-placed signs will have the new features right away.