vcc_acl: Add +fold(-report) sub-flag omit +fold warnings

[nigoroll]

With a lot of folding going on, the warnings can easily bury more relevant CLI output.

[nigoroll]

bugwash with @bsdphk : +silent sounds too general, as if we would suppress +log or other errors.
We discussed several options, and ended with the usual "write the docs first to see how it works" (which I had done for +silent, but did not see the problem with it).

So I have adjusted the docs but also the rest of the PR, just because it's easy enough.

[nigoroll]

I have updated the patch to the last suggestion with the sub-flag in parenthesis, but I stuck with report, because the fold reports are just that, not really warnings. warn was brought back on the table as a generic flag, but this is a sub flag for fold.

From the docs:

Exactly one sub-flag is supported following fold in parenthesis:

• +fold(+report) - Fold with reporting (default)

  Report about folding as described above

• +fold(-report) - Fold without reporting

  Enable folding, but do not output folding-related warnings during VCL
  compilation

The report sub-option is only valid with +fold.

[walid-git]

I think we should stop the loop if we failed in vcc_parseAclFold, otherwise we might get double error reporting. Maybe we can turn the loop into a while (!tl->error) ?