Fix GeneralizedTime fractional seconds implementation

[sjaeckel]

@karel-m #182 (comment)

Ad implementation I would simply store nanoseconds (if we can afford one "ulong32") like:

- unsigned fs, /* fractional seconds */
+ ulong32  ns, /* nano second */

but I have not think too much about consequences.

Nano seconds won't fit in a ulong32 but micro seconds would be fine.

When I think about the implementation I'd still prefer my bitmasking proposal #182 (comment) ... :-D

But regarding the usage I think it'd make most likely sense to provide the API with microseconds instead of the bitmasking proposal...