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

ok kettenis@ millert@ deraadt@

Declare __p1evll and __polevll protos in math_private.h instead of
redeclaring each time in ld{80,128}. Pointed out by theo.

- 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.

Finalize work on the math library. It's time to do this monster
commit, and deal with problems (if any) in tree.

Note that this adds the following functions. Ports with hacks might
need adjustments.

nexttoward(3), fma(3), nexttowardf(3), fmaf(3), acoshl(3), asinhl(3),
atanhl(3), coshl(3), sinhl(3), tanhl(3), expl(3), expm1l(3), logl(3),
log10l(3), log1pl(3), log2l(3), modfl(3), cbrtl(3), hypotl(3),
powl(3), erfl(3), erfcl(3), lgammal(3), tgammal(3), ceill(3),
floorl(3), lrintl(3), llrintl(3), roundl(3), lroundl(3), llroundl(3),
truncl(3), fmodl(3), remainderl(3), remquol(3), nextafterl(3),
nexttowardl(3), fmal(3).

With this commit, our library implements all functionality required
by C99. Documentation bits will follow.