Previously, we would only look for the first ever match.
To support multiple matches, we now save the start offset
relative to the ptr argument for the next invocation.

... or the highest match

The main reason is an upcoming improvement which needs workspace.

But also using workspace here probably does not come with too relevant
an advantage, because regular expression matching and copying are
already expensive, and the need to copy should be the exception.

This is in preparation of an upcoming refactor to heap space.

Add filters: regex objects to perform substitutions on bodies

See merge request !3

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