Revert the default for font bolding style back to using colours rather

Revert the default for font bolding style back to using colours rather
than fonts. I broke this in r9559 when I added the option for 'both',
because the internal representation got offset by one so as to change
from a boolean to two bitfields and I must have confused myself about
what the default should be.

git-svn-id: http://svn.tartarus.org/sgt/putty@10008 cda61777-01e9-0310-a592-d414129be87e