Fix --keep-old-files option.

The regression was introduced by 8f390db9.  This patch implements additional
option --skip-old-files, which silently skips members which would cause
writing over existing files, and restores --keep-old-files to its traditional
behavior.

* NEWS: Update.
* configure.ac: Update.
* doc/tar.texi: Document the changes.
* src/common.h (SKIP_OLD_FILES): New old_files mode.
* src/extract.c (maybe_recoverable): Restore KEEP_OLD_FILES behavior.
Handle SKIP_OLD_FILES.
* src/tar.c: New option --skip-old-files.
* tests/extrac18.at: New file.
* tests/extrac19.at: New file.
* tests/Makefile.am: Add new test cases.
* tests/testsuite.at: Likewise.

src/common.h

@@ -183,6 +183,7 @@ enum old_files
   OVERWRITE_OLD_FILES,        /* --overwrite */
   UNLINK_FIRST_OLD_FILES,     /* --unlink-first */
   KEEP_OLD_FILES,             /* --keep-old-files */
+  SKIP_OLD_FILES,             /* --skip-old-files */
   KEEP_NEWER_FILES	      /* --keep-newer-files */
 };
 GLOBAL enum old_files old_files_option;

src/extract.c

@@ -642,11 +642,14 @@ maybe_recoverable (char *file_name, bool regular, bool *interdir_made)
 
       switch (old_files_option)
 	{
-	case KEEP_OLD_FILES:
+	case SKIP_OLD_FILES:
 	  WARNOPT (WARN_EXISTING_FILE,
 		   (0, 0, _("%s: skipping existing file"), file_name));
 	  return RECOVER_SKIP;
 
+	case KEEP_OLD_FILES:
+	  return RECOVER_NO;
+	  
 	case KEEP_NEWER_FILES:
 	  if (file_newer_p (file_name, stp, &current_stat_info))
 	    break;

src/tar.c

@@ -328,6 +328,7 @@ enum
   SHOW_DEFAULTS_OPTION,
   SHOW_OMITTED_DIRS_OPTION,
   SHOW_TRANSFORMED_NAMES_OPTION,
+  SKIP_OLD_FILES_OPTION,
   SPARSE_VERSION_OPTION,
   STRIP_COMPONENTS_OPTION,
   SUFFIX_OPTION,
@@ -452,7 +453,11 @@ static struct argp_option options[] = {
   {"remove-files", REMOVE_FILES_OPTION, 0, 0,
    N_("remove files after adding them to the archive"), GRID+1 },
   {"keep-old-files", 'k', 0, 0,
-   N_("don't replace existing files when extracting"), GRID+1 },
+   N_("don't replace existing files when extracting, "
+      "treat them as errors"), GRID+1 },
+  {"skip-old-files", SKIP_OLD_FILES_OPTION, 0, 0,
+   N_("don't replace existing files when extracting, silently skip over them"),
+   GRID+1 },
   {"keep-newer-files", KEEP_NEWER_FILES_OPTION, 0, 0,
    N_("don't replace existing files that are newer than their archive copies"), GRID+1 },
   {"overwrite", OVERWRITE_OPTION, 0, 0,
@@ -1544,7 +1549,7 @@ parse_opt (int key, char *arg, struct argp_state *state)
       /* Don't replace existing files.  */
       old_files_option = KEEP_OLD_FILES;
       break;
-
+      
     case 'K':
       starting_file_option = true;
       addname (arg, 0, true, NULL);
@@ -1674,6 +1679,10 @@ parse_opt (int key, char *arg, struct argp_state *state)
       sparse_option = true;
       break;
 
+    case SKIP_OLD_FILES_OPTION:
+      old_files_option = SKIP_OLD_FILES;
+      break;
+
     case SPARSE_VERSION_OPTION:
       sparse_option = true;
       {