cksum/hashsum: refactor the common code.

Summary of the change:
* Move the common code into checksum
* Create a structure HashAlgorithm to handle the algorithm (instead of the 3 variables)
* Use the same function for cksum & hashsum for --check (perform_checksum_validation)
* Use the same for function for the hash generation (digest_reader)
* Add unit tests
* Add integration tests
* Fix some incorrect tests

tests/by-util/test_cksum.rs

@@ -554,6 +554,15 @@ fn test_blake2b_512() {
         .arg("checksum")
         .succeeds()
         .stdout_contains("f: OK");
+
+    scene
+        .ucmd()
+        .arg("--status")
+        .arg("--check")
+        .arg("checksum")
+        .succeeds()
+        .stdout_contains("")
+        .stderr_contains("");
 }
 
 #[test]
@@ -1168,3 +1177,18 @@ fn test_unknown_sha() {
         .fails()
         .stderr_contains("f: no properly formatted checksum lines found");
 }
+
+#[test]
+fn test_check_directory_error() {
+    let (at, mut ucmd) = at_and_ucmd!();
+
+    at.mkdir("d");
+    at.write(
+        "f",
+        "BLAKE2b (d) = 786a02f742015903c6c6fd852552d272912f4740e15847618a86e217f71f5419d25e1031afee585313896444934eb04b903a685b1448b755d56f701afe9be2ce\n"
+    );
+    ucmd.arg("--check")
+        .arg(at.subdir.join("f"))
+        .fails()
+        .stderr_contains("cksum: d: Is a directory\n");
+}