Homogenize include guards
Use include <> in header files

Header files should use the 'include <>' syntax instead of 'include ""'
as they are really searching for "system headers".
Include atf-c.h and atf-c++.hpp in test programs

Instead of including the macros.h or macros.hpp file in test programs,
include the public header file directly to better represent the
expected use of ATF's APIs.
Remove unnecessary run-time header checks

Drop all HEADER_TC uses to validate that individual, internal header
files can be included without prerequisites.  Our changes in
fe71aabbe5e75628282a44621cdd15414212b90e do the exact same thing at
compile time in a much simpler way.

For now, keep these checks for the public header files given that we
cannot easily kill the boilerplate required to support HEADER_TC due
to other tests we have for the public macros.
Remove empty and internal test program
Remove package name from file headers

It's pointless to mention ATF in every single file header.  Just keep
the license text, which is already long enough.