Reduce libm's exports and make internal calls go direct.
All dependencies on libc are now via reserved/standardized names.

ok kettenis@ millert@ deraadt@

- Make constants const.
- Fix aliasing issue in s_frexpf.c; spotted by gcc. Many other
ones were fixed in NetBSD long ago, but this one was forgotten. It
looks like recently they fixed it too.

For intermediate computations use single or extended counterparts
where appropriate, to avoid precision loss.

Finalize work on complex math routines, now that we have the
extended-precision support. Mostly from Cephes.