Rename atf-*-api(3) to atf-*(3)

Drop the -api suffix from the atf-c-api(3), atf-c++-api(3) and
atf-sh-api(3) manual pages for simpler discoverability.

Install backwards compatibility symbolic links.
Rename bconfig.h to config.h

The config.h file generated by autoconf was originally named bconfig.h
to avoid naming collisions with the config module provided by atf-c.
However, the config module was removed in 448d8cf so there is no need
for this oddity any longer; get rid of it.
Fix all includes to be relative to the top dir

We know that our Makefiles are passing -I flags for the top source
directory and the top build directory.  Make sure our includes are
relative to those for clarity and consistency.
Include the module's own header file first

Move the inclusion of the header file corresponding to the module's
.c file or test program to the very beginning.  This is an easy way
to ensure that the header files are self-contained, as compilation
will fail if they aren't.
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.
Fix hack to not distribute atf-c/defs.h

The reason atf-c/defs.h was still being included in the distfile even
though it was listed as "nodist SOURCES" was that the file was still
listed as "dist HEADERS".  Make sure the latter is also nodist.
Merge branch 'cleanup-includes'

Various cleanups to include directives and to file headers.