Use more libm.

Use more libm.

adj360() is a modulo operation: use fmod(3) to simplify things.

Adding 0.5 to the phase to steer implicit truncation is a hack:
use lround(3) to simplify things.

Product of a discussion with pjanzen@/jca@/tb@ regarding floating-point
rounding and the importance of correctly classifying a gibbous moon.

ok pjanzen@ jca@ tb@