Fix logged request size (%http::>st) and other size-related %codes.

Fix logged request size (%http::>st) and other size-related %codes.

The %[http::]>st logformat code should log the actual number of
[dechunked] bytes received from the client. However, for requests with
Content-Length, Squid was logging the sum of the request header size and
the Content-Length header field value instead. The logged value was
wrong when the client sent fewer than Content-Length body bytes.

Also:

* Fixed %http::>h and %http::<h format codes for icap_log. The old code
  was focusing on preserving the "request header" (i.e. not "response
  header") meaning of the %http::>h logformat code, but since ICAP
  services deal with (and log) both HTTP requests and responses, that
  focus on the HTTP message kind actually complicates ICAP logging
  configuration and will eventually require introduction of new %http
  logformat codes that would be meaningless in access.log context.

  - In ICAP context, %http::>h should log to-be-adapted HTTP message
    headers embedded in an ICAP request (HTTP request headers in REQMOD;
    HTTP response headers in RESPMOD). Before this change, %http::>h
    logged constant/"FYI" HTTP request headers in RESPMOD.

  - Similarly, %http::<h should log adapted HTTP message headers
    embedded in an ICAP response (HTTP request headers in regular
    REQMOD; HTTP response headers in RESPMOD and during request
    satisfaction in REQMOD). Before this change, %http::<h logged "-" in
    REQMOD.

  In other words, in ICAP logging context, the ">" and "<" characters
  should indicate ICAP message kind (where the being-logged HTTP message
  is embedded), not HTTP message kind, even for %http codes.

  TODO: %http::>h code logs "-" in RESPMOD because AccessLogEntry does
  not store virgin HTTP response headers.

* Correctly initialize ICAP ALE HTTP fields related to HTTP message
  sizes for icap_log, including %http::>st, %http::<st, %http::>sh, and
  %http::>sh format codes.

* Initialize HttpMsg::hdr_sz in HTTP header parsing code. Among other
  uses, this field is needed to initialize HTTP fields inside ICAP ALE.

* Synced default icap_log format documentation with the current code.