Improved isUnsigned to check attribute minimum more carefully

[seanm]

The previous implementation checked the validation predicate for a string containing literally ">= 0". But if an attribute's minimum was, for example, 1 instead of 0, the test would fail because the string would be ">= 1". But when a minimum is 1 we still want to use unsigned, but weren't getting unsigned because of this.

Updated the code to instead look for the string "SELF >=" and then inspect the number that comes after (if any). Then we can in fact correctly check that the minimus is >= 0, thus allowing unsigned to be used in those cases too.

[seanm]

Thanks @rentzsch !

After all these years, we are updating from mogen 1.27. :)