Emit GeoIP cookie using dedicated Set-Cookie header

If multiple Set-Cookie headers are set, VRT_GetHdr will only get the first, but
VRT_SetHdr will clobber all of them. Varnish provides no facility for combining
multiple instances of a particular header. We hack around that by creating a
header with a unique name ("Xet-Cookie"), and then changing the name to
"Set-Cookie", exploiting the fact that Varnish lays out the complete header
(name and value) as a contiguous block of memory.

Change-Id: I1cf419a845a89eb5b46faafe881e145e7c52ea1c