* Add a ParseZone test for $GENERATE.
* Add a test for modToPrintf used by $GENERATE.
* Correctly handle $GENERATE modifiers.
As per http://www.zytrax.com/books/dns/ch8/generate.html, the width and type (aka base)
components of a modifier are optional. This means that ${2,0,d}, ${2,0} and ${2} are
valid modifiers, however only the first format was previously permitted. Use default
values for the width and/or type if they are unspecified in the modifier.
Joel Sing