Correctly computing the needed amount to scroll down when softwrap is on and there are overlong lines. (from nano commit #4632)

Original nano commit: 4632
http://svn.savannah.gnu.org/viewvc?view=rev&root=nano&revision=4632
Original nano commit message:
	Correctly computing the needed amount to scroll down
	when softwrap is on and there are overlong lines.
Correcting scrolling behavior without softwrap (adapted from nano commit #4633)

Original nano commit: 4633
http://svn.savannah.gnu.org/viewvc?view=rev&root=nano&revision=4633
Original nano commit message
	Trimming some redundant code, and correcting scrolling behaviour
	without softwrap.  And adding a few more NANO_TINY wrappings.