Reduce testHttpRange dependencies (#2148)

Reduce testHttpRange dependencies (#2148)

This drastic reduction in testHttpRange dependencies is made possible by
splitting HeaderTools code into several parts:

* Header-mangling code that pulled in many heavy dependencies but was
  unused by testHttpRange code. This high-level code does not belong to
  libhttp, so it was left in src/ (see HeaderMangling.cc).

* Simple header manipulation functions without heavy dependencies; some
  used by testHttpRange code. This low-level code should eventually be
  moved to libhttp, but it was left in HttpHeaderTools.cc for now
  because libhttp itself is currently bloated with heavy dependencies --
  linking with libhttp requires linking with or stubbing a lot of
  code unrelated to testHttpRange.

* Definition of httpHeaderParseQuotedString() and a few other functions
  declared in HttpHeader.h were moved to HttpHeader.cc to improve
  declaration/definition/stubs affinity and avoid linking errors:
  testHttpRange now depends on stub_HttpHeader.cc that has stubs for
  these functions.

* httpHeaderMaskInit() and getStringPrefix() are only used by
  HttpHeader.cc, so they were moved to HttpHeader.cc and made static.

Moved code was unchanged. A few comments were relocated to better match
Squid coding style. A few STUBs were added/adjusted as needed.

TODO: Check the remaining two tests that still include header mangling
code: testHttpRequest and testCacheManager.