dc(1): fix modulo operations with fractional inputs

Our dc(1) has never correctly calculated remainders with fractional inputs.
Both bmod and bdivmod seem to have copy/pasted code from bdiv, which results
in the remainder having the wrong output scale.

PR: 162495
Reported by: anonymous
Reviewed by: pfg
Differential Revision: https://reviews.freebsd.org/D13390