Backend get-if-modified-since

[Nils Goroll]

Hi,

> If not, I will implement this feature.

OK, because there were no answers, we're going to implement this feature. As
with almost all features/fixes I've implemented, patches will be submitted to trac.

I'll check first if this can be done in VCL, but I remember to have thought
about this already and last time I was of the opinion that the feature needs to
get implemented in C.

Thanks, Nils