libutil: Backward compatibility for expand_number()

Reimplement expand_number() in terms of expand_unsigned(), which takes
a pointer to uint64_t like expand_number() did before. Provide a macro
that picks the correct version based on the type of the argument.

Fixes: 2e0caa7c7e14
Reviewed by: jhb
Differential Revision: https://reviews.freebsd.org/D51723