add a commercial support reference
Add license to vxp_test.c
Document y (years) time unit
Add y (years) time unit support to VCL

Follows the change in the std vmod a while ago.
White space nits
Make sure attempts to talk (draft-)HTTP/2.0 don't confuse anybody.
Add VRT_MAJOR_VERSION and VRT_MINOR_VERSION as discussed on the Stockholm VDD.

Renovate and simplify the compile-time binding between VMODs, VCC
and varnishd.