libutil: Backward compatibility for expand_number()

[des]

Summary

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

[des]

typo

[jhb]

Looks good to me

[kib]

Don't you need to export expand_unsigned in version def?

lib/libutil/libutil.h

93

_Generic use should be braced by the C dialect check, since this is a public header.

[des]

complete

[des]

nit

[des]

documentation