digest: refactor sum(1) into digest.c

digest: refactor sum(1) into digest.c

Since digest will be providing all digest functionality,
refactor sum.c into it.

* po/POTFILES.in: sum.c no longer has translatable strings so remove.
* src/digest.c: Call out to new stream interfaces in sum.c
* src/local.mk: Adjust sources for the sum binary.
* src/sum.c: Provide a stream interface for BSD and SYSV digests.
* src/sum.h: A new file to declare the exported functions in sum.c