vi status line fixes

[Paul Fox]

i got no comments yea or nay, so i've committed this, in svn r11477.

paul

 > 
 > the main effects are that a) the status line should always be
 > refreshed properly, b) the status line is a little shorter than
 > it used to be ("I" instead of "--INSERT--"), c) the status line
 > will be truncated if it doesn't fit on the screen, and d) if the
 > screen is too narrow for an error or transient status message
 > (from psb() or psbs()), then that message will be followed by a
 > "Hit Return" prompt.  (it wasn't until i did this last bit that
 > the size grew.  with this, these changes add about 150 bytes.)
...

=---------------------
 paul fox, pgf at brightstareng.com