Changes to HTTP request and reply parsing classes, started by Alex,

Changes to HTTP request and reply parsing classes, started by Alex,
further hacked by DW.  These are forward-ported from ICAP development
on sourceforge.

Alex's original commit message:

- Moved parts common to HttpReply and HttpRequest to HttpMsg. HttpMsg is
  needed to be able to pipe HTTP messages using MsgPipe without knowing
  whether the message being piped is a request or response. It is also
  handy for keeping ICAPXaction mostly independent from the ICAP request
  method (and, hence, the primary HTTP message kind). Moving previously
  duplicated code to a single location is supposed to be good long-term.
  More polishing work is needed (e.g., protocol and protoPrefix need
  to be merged and so can be lastmod and last_modified).

- Added reset/parse/pack methods to parse and pack HTTP messages without
  knowing their direction/kind. HttpRequest::parse() is not implemented
  yet but we have code that should be reusable for that.

DW's additional changes

- HttpRequest didn't really have any parsing methods or state.
  Moved the HttpReply parsing methods to HttpMsg and created a
  virtual function for parsing the start line (request line or
  status line).

- Added full_uri flag to packInto() method because ICAP needs
  full URIs in encapsulated requests.