Add crypt.[ch] and update md5sum and sha1sum

Factor out the code from md5sum and sha1sum into a util function.

Use FILE * instead of a file descriptor.  This will make it a bit
easier/more consistent when we implement support for the -c option.

crypt.h

@@ -0,0 +1,10 @@
+struct crypt_ops {
+	void (*init)(void *);
+	void (*update)(void *, const void *, unsigned long);
+	void (*sum)(void *, uint8_t *);
+	void *s;
+};
+
+int cryptsum(struct crypt_ops *ops, FILE *fp, const char *f,
+	     uint8_t *md);
+void mdprint(const uint8_t *md, const char *f, size_t len);