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.
Letting tgammal() modify signgam has two disadvantages:
- It breaks valid code that assumes that the value of signgam is not
clobbered by calls to tgammal().
- It makes this function depend on the presence of signgam. signgam is
an X/Open System Interface. It is not part of the C standard.
