VIP4: Restrict VMOD function call sites

[Dridi Boukelmoune]

Hi,

As instructed last Monday after the bugwash, I have created a new VIP.
We started a discussion on restricting VRT functions usage during
compilation of VMODs, I suggested a syntax to do that for VMOD
functions.

I interpreted phk's request for me to do a write up as part of the VIP
process and came up with a slightly different suggestion.

https://github.com/varnishcache/varnish-cache/wiki/VIP4%3A-Restrict-VMOD-function-call-sites

Best Regards,
Dridi