[MERGE] Remove old .h file tests from unit-testing

3.1 Cleanups pt 1: Add testheaders.sh script

This script automatically tests all .h files in any directory it is passed
(relative to curent make directory) using the compiler options it is also passed.
It should be integrated in to the unit-testing in every Makefile.am so that
it provides dependancy tree testing of all .h without any additional work by
developers, maintainers, or testers.

3.1 Cleanups, pt 2: Add automatic validation and fix existing problems.

- puts testheaders.sh script to use in many Makefile.am
  (tried for all subdirs tested, but some may have escaped)

- fixes many include problems where .h were not self-sufficient.
  They required .cc or other .h files using them to include additional .h
  Now any given .h will include all objects it requires for compiling.
  (linking is still not guaranteed by this alone)

- References SQUIDHOSTNAMELEN properly as defined by RFC 2181.
  Now also split into a proper rfc2181.h for use.
  (obsoleting many dependencies on squid.h all by itself)

- Migrates dlink_* (squid custom double-linked-list code) into new object.
  (releasing many dependencies on squid.h, structs.h, and protos.h)

- Migrates the SquidConfig* types into SquidConfig.cc, now built into libsquid.la
  (releasing many dependencies on structs.h, and globals.*)

- removes squid.h includes from many unit-tests
  (releasing them from link-dependencies, .o links not yet cleaned up)