Ticket #3183: diffviewer line drawing fix.

The "topmiddle" and "bottommiddle" characters are defined incorrectly
in many skins. This is because the correct definition wouldn't work
with S-Lang build. The only place these characters are used is the
diffviewer if either the +/- signs or the line numbers are shown,
enabled by pressing S or L. The correct definition of these characters
currently show up as 'v' and 'w' characters.

The fix makes the diffviewer use tty_print_alt_char() as it is being used
in other parts of the code, and hence it fixes the bug.
It also fixes the skin definitions.

Signed-off-by: Andrew Borodin <aborodin@vmail.ru>
Merge branch '3183_diffviewer_lines'

* 3183_diffviewer_lines:
  Ticket #3183: diffviewer line drawing fix.