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.