Migration to PCRE2

[Dridi Boukelmoune]

We just completed a migration of our regular expression engine from
PCRE to PCRE2.

https://github.com/varnishcache/varnish-cache/pull/3635

There may be subtle differences that we would be eager to learn about.
We discovered for example that PCRE2 will reject unknown escape
sequences like "\T" whereas PCRE would interpret it as "T". There
might be an option flag to loosen this rule but we decided to stick to
the strict behavior.

We generally expect that potential breaking changes in the regular
expression syntax should be minor and trivial to correct.

If you wish to test this change scheduled for the next biannual
release in September, it is available in the git master branch. Linux
users may also find prebuilt weekly packages at the usual address.

https://packagecloud.io/varnishcache/varnish-weekly

--
Dridi