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.
Put external headers before internal ones. While there, replace a lot of
occurences of "openlibm.h" with <openlibm.h>. It should be thought of as
an external header, as it is installed along with the library.
