Add cryptmain() and factor out the code from the crypt tools

crypt.h

@@ -5,6 +5,8 @@ struct crypt_ops {
 	void *s;
 };
 
+int cryptmain(int argc, char *argv[],
+	      struct crypt_ops *ops, uint8_t *md, size_t sz);
 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);