[PATCH] meta_header option

Hi all,

This patch adds meta_header option to squid.conf. It is similar to
adaptation_meta but is applied after all adaptation and before logging.
Values of named meta headers can be logged using %{name}meta_header macros.

    meta_header name value acl ...
    logformat myFormat ... %{name}meta_header ...

This option may be initially used to log custom information about the
master transaction. For example, an admin may configure Squid to log
which "user group" the transaction belongs to, where "user group" will
be determined based on a set of ACLs and not [just] authentication
information.

From user point of view, adaptation_header sets/implies meta_header
(i.e., setting adaptation_header is sufficient to be able to log it
using %meta_header) but the meta_header option itself (if any) is
evaluated later, so it has no effect on ICAP headers.

This is a Measurement Factory project