New VCL_BLOB type

[Poul-Henning Kamp]

As agreed at the VDD in Berlin, I have added a "BLOB" type to the VCC.

It can only be used as return value and argument to VMOD functions.

I've reused the vmod_priv structure as carrier for BLOBs and added
a length field to it for this purpose.

Examples in libvmod_debug and m00012.vtc

Please provide feedback ASAP-ish if this implementation will not
work for the usecases we talked about (vmod-digest etc.) so we
do not cast a bad implementation in stone for 4.0-R.

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