This is in essence the same VMOD so migrating from one to the other
should result in no VCL change, except for the removal of DEPRECATED
cookie.filter_except().

The test suite was refreshed to take advantage of recent features and
mostly operates in vcl_synth as a result, not wasting cycles bringing
needless backends up.

The C code and VCC descriptor were also improved for better code style
compliance and documentation consistency, without changing the VMOD's
behavior. On the C side actual changes mostly consisted in adding missing
assertions but there were notable changes:

- usage of VRE_Free instead of free (with test case)
- WS_VSB_* facility usage for the one use case

There are other opportunities for improvement, like usage of more modern
facilities like VRT_fail, or a named Lck instead of a plain mutex.

Closes #3184