The arguments we pass to the VCL and VRT code is getting out of hand
and I've already forgot about them one important place already.

Introduce a "VRT_context", which is a container holding all the
bits we want to pass to VCL and VRT.

Implement it first for VRT_count(), which is, BTW, how I realized I
had forgotten to pass all the new arguments to user defined
functions.
Make the VRT_[GS]etHdr() take vrt_ctx arg.
Move acls to vrt_ctx
Replace the newly introduced "ws" argument to VCL/VRT, with the
even newer vrt_ctx argument.
Replace the newly added busyobj argument with vrt_ctx.

This actually makes some of the macro-expansions in cache_vrt_var.c
simpler and saner...
Make all the VCL variables controlled by generate.py use vrt_ctx.
Replace the remaining instances of the the "req" argument in VCL/VRT/VMOD
with vrt_ctx arguments.
Remove the wrk argument from VCL/VRT, now everything we need is
encapsulated in the vrt_ctx.
Remove unused function VRT_MkGethdr()
Make VRT_String() return a const char *
Don't let the form of the argument to hash_data() leak into the
production of the hash-key.

Fixes	#1296