fmod, fmodl: Fix computation for large quotients x / y. * lib/fmod.c: Completely rewritten. * lib/fmodl.c (fmodl): Use implementation of fmod.c with USE_LONG_DOUBLE. * modules/fmod (Depends-on): Add isfinite, signbit, fabs, frexp, ldexp, isnand. Remove fma. * modules/fmodl (Depends-on): Add float, isfinite, signbit, fabsl, frexpl, ldexpl, isnanl. Remove fma. * m4/fmod.m4 (gl_FUNC_FMOD): Update computation of FMOD_LIBM. * m4/fmodl.m4 (gl_FUNC_FMODL): Update computation of FMODL_LIBM.