When parsing git configs, support all the documented ways to write true and false...

When parsing git configs, support all the documented ways to write true and false, including "yes", "on", "1", etc.

This change does impact git-annex config
eg "git annex config --set annex.addunlocked on"
will store "on" and new git-annex will understand that value, while
old git-annex will error:
git-annex: bad annex.addunlocked configuration in git annex config:
Parse failure: near "on"
That seems acceptable.

Not special remote configs that are only documented as =true or =false
however. Having git-annex support other values for those would break
backwards compatability when used with old versions of git-annex. And
older versions ignore invalid special remote configs.. That would not
be a good combination.