[PATCH] Add request_header_add option and [request|reply]_header_* manglers fixes

This patch:

1)add request_header_add, a new ACL-driven squid.conf option that allows
addition of HTTP request header fields before the request is sent to the
next HTTP hop (a peer proxy or an origin server):
  request_header_add <field-name> <field-value> acl1 [acl2] …
where:
    * Field-name is a token specifying an HTTP header name.
    * Field-value is either a constant token or a quoted string
containing %macros. The following field-value macros are supported:
%LOGIN, %SRC, %DST, %user_cert_subject, %user_ca, %%
    * One or more Squid ACLs may be specified to restrict header
insertion to matching requests. The request_header_add option supports
“fast” ACLs only.

2) Fix the [request|reply]_header_[access|replace] configuration
parameters to support custom headers. Currently the user has the option
to remove/replace/allow all custom headers using the "Other" as header
name, but no one or some of the custom headers.

This is a Measurement Factory project.