utils: Checkbox with U+2612 and U+2610 instead of hexagons

U+2612 BALLOT BOX WITH X and U+2610 BALLOT BOX are both Unicode since 1.1. 
They are closer to both the symbols used for win32 and the
"checked" and "checkbox" language in the getCheckbox comment.  Plus I didn't
have a monospaced font for Inquirer's previous U+2B22 BLACK HEXAGON or U+2B21
WHITE HEXAGON, which have only been in Unicode since 5.0.

The ballot box glyphs are in DejaVu Sans Mono [1].  I have the hexagon glyphs
in a number of fonts including Unifont [2], but not in a monospace font. 
Installed fonts are obviously subjective, but I expect the older v1.1 code
points will have better coverage than the newer 5.0 code points.

[1]: http://dejavu-fonts.org/
[2]: http://unifoundry.com/unifont.html