If the optional ``asfilter`` parameter to ``re.regex()`` is true, the
vmod registers itself as a Varnish Fetch Processor (VFP) for use in
`beresp.filters` and as a Varnish Delivery Processor (VDP) for use in
`resp.filters`. In this setup, the `xregex.substitute_match()` and
`xregex.substitute_all()` methods can be used to define replacements
for matches on the body.

Example:

	    sub vcl_init {
		new reiher = re.regex("r(ei)h(er)", asfilter = true);
	    }
	    sub vcl_deliver {
		unset req.http.Accept-Encoding;
		set resp.filters += " reiher";
		reiher.substitute_match(1, "czapla");
		reiher.substitute_match(2, "\1\2");
		reiher.substitute_match(0, "heron");
	    }

Ref varnish-cache 478e310a7d2c168e1c361d6f34291177cbc2a889

Code taken from varnish-cache

... such that it can optionally be called before match()

This is in preparation for body matching.

This is in preparation for body matching.

See varnish-cache 7f28888779fd14f99eb34e50f6fb07ea6bbff999

The VCL_REGEX type requires that the pattern consist of constant strings only.

In contrast, with a STRING argument and compilation in this vmod, we can support
patterns to be constructed dynamically in vcl_init {}, which is a relevant
advantage.

I did consider to support both a STRING and a VCL_REGEX argument (both being
optional, requiring at least one to be set) but found no compelling reason
for using the VCL_REGEX type, because the pattern is checked anyway and it
does not make a relevant difference if that check happens at the VCC or
the vcl_init stage.

This reverts commit 0ee2f4ae.

Based on the VRE_unpack() PR in
https://github.com/varnishcache/varnish-cache/pull/3657

As discussed with Geoff, we might still switch to VRE_capture()
once it is cooked, see https://github.com/varnishcache/varnish-cache/pull/3659

When we copy onto ovector[MAX_OV_USED] == ovector[MAX_MATCHES * 2], we
want to make sure that we do not exceed the destination, not the source.

It could infer that the result of a non-NULL cast of task->priv can not
be null, but it doesn't. Sigh.

Ref:
https://github.com/varnishcache/varnish-cache/commit/681c11998ec56dc4460a30585210f95a7850ea3b