Fix some tests on Win9x.

I tested without the ES_MULTILINE style, and found that wrapping
doesn't occur where it normally would.

When all the text fits on the screen, the scrollbars are not shown from
EM_SHOWSCROLLBAR.  The message instead adds support for the specified
scrollbar when lParam is TRUE, so that the scrollbar will be shown when
sufficient text is in the document.

The scrollbar visibility can be changed from SetScrollRange or
SetScrollInfo, but the visiblity that is a result of these calls are
not consistent with the calculation made by richedit controls to
decide whether to show or hide the scrollbars.

The vertical scrollbar is not shown when the ES_MULTILINE style isn't
used, unless ES_DISBALENOSCROLL is also used.

The buffer is to be overwritten anyway, so there's no need to preserve its
contents.  Also, the old code leaked if the realloc were to fail.  All in all,
free-then-alloc is simpler.