[master] 169e162 Enforce that VCL names must be C-language identifiers ([A-Za-z][A-Za-z0-9_]*)

[Dridi Boukelmoune]

> If '-' is in widespread use, we should not break it.
>
> I asked on IRC earlier, but nobody reacted...

I don't really care, I merely pointed out that it's a breaking change
and we should not forget packaging before the release. Downstream or
in-house tools like the varnish agent may catch up later, but people
should be aware when reading the upgrade notes that VCL names may
break.

After all, this is 5.0 :)

I'm also for having C-idents to match VCL labels naming rules, and
migrating from '-' to '_' should be enough for users (for this character at
least).

Cheers