This is a bit more consistent with the naming of the other header files
(openlibm_complex.h and openlibm_fenv.h). Re-add an openlibm.h header
that includes all of the public headers as a shorthand.
Fix up all of the source files to include <openlibm_math.h> instead of
<openlibm.h>. While there, fix ordering of the includes.
I am currently working on building openlibm against stock copies of
<math.h>, instead of the openlibm.h header. It seems that a C compliant
<math.h> header can be used as a drop-in replacement for openlibm.h,
with the exception that it lacks cpack*().
In FreeBSD SVN r275819 I patched up the math library by replacing
cpack*() by CMPLX*(). That way many functions become less tied to the
intrinsics of the math library. Make the same change to openlibm.
