(md5_digest): Call md5_final and md5_init.

(md5_final): Declared static.
sha1.c, sha256.c: Analogous changes.

Rev: src/nettle/md5.c:1.4
Rev: src/nettle/md5.h:1.4
Rev: src/nettle/sha.h:1.2
Rev: src/nettle/sha1.c:1.6
Rev: src/nettle/sha256.c:1.2