Fix vertical lines in Putty with UTF-8 graphics

[proski]

Putty doesn't show VT100 line drawing characters in the UTF-8 mode by
default. Putty 0.70 and older doesn't even have that option.

For the vertical display separator, use '|' as background, then output
the vertical line character on every line.

For the line number separator, use draw_graphic() in the default mode
only. Use draw_chars() in the ASCII and UTF-8 modes.

Add a test for vertical lines. The display separator is always captured
as a line of '|' characters. The line number separators are captured
correctly. 'x' corresponds to ACS_VLINE.

Closes #981

[proski]

Fixed an issue with the vertical window separator not redrawing correctly after cycling graphics modes with ~ on Ubuntu 14.04. Using a simple one-line approach to iterate over lines without using getmaxyx() ("it's better to ask forgiveness than permission").

[proski]

• Using switch without default. If a new graphics mode is used, the compiler would warn about a unhandled option, which is a good thing.
• Made the line number separator code closer to the graph output code. ASCII and UTF-8 output strings, DEFAULT draws symbols.

[proski]

• In the line number separator code, return false for the unsupported graphics setting to fix a compiler error.
• In the display separator code, use pipe (|) as background in the UTF-8 mode. It gets overwritten anyway, but the logic becomes simpler.

[proski]

Added a test for vertical lines in all graphic modes. Unfortunately, the vertical display separator is always output as | symbols by save-display. However, the line number separators are captured correctly.

[proski]

Changed redraw_display_separator() to be more uniform. The separator is now drawn in a loop in all graphic modes. wbkgd() is replaced with wbkgdset(), so the curses library can save some time clearing the window.

Using background to draw the separator is a nice trick, but it won't work in the UTF-8 mode without using cchar_t and introducing compatibility issues. It's better to do the same thing in all modes unless there are reasons to do it differently.

[jonas]

Thanks a lot. Really great to have the test.