Parser-NG: Convert the ICAP read buffer to an SBuf.

* Remove the double-buffering hack used to comm_read() ICAP responses as
  c-string then convert to MemBuf for parsing.

* Revert the HttpMsg parser API from MemBuf to c-string parameters.
  The internals did not make much use of the MemBuf abilities and it is
  simpler to retrieve c-string values directly from an SBuf than to go
  via a MemBuf conversion.