Add request_header_add option

Add request_header_add option

This patch:

- Add request_header_add, a new ACL-driven squid.conf option that
  allow 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. In theory, all of the logformat codes can be used as %macros.
       However, unlike logging the transaction may not yet have enough
       information to expand a macro when the new header value is needed.
       The macro will be expanded into a single dash ('-') in such cases.
       Not all macros have been tested.

     * One or more Squid ACLs may be specified to restrict header insertion to
       matching requests. The request_header_add option supports fast ACLs only.

- Add the %ssl::>cert_subject and %ssl::>cert_issuer logformating codes which
  prints the Subject field and Issuer field of the received client SSL
  certificate or a dash ('-').

This is a Measurement Factory project.