Warn about existing files in keep-old-files mode (complements 8f390db9).

This is controlled by "existing-file" warning code, which is off
by default.

* src/common.h (WARN_EXISTING_FILE): New warning code.
(WARN_VERBOSE_WARNINGS): Add WARN_EXISTING_FILE.
* src/extract.c (maybe_recoverable): warn about existing file
if WARN_EXISTING_FILE is set.
* src/warning.c: New warning type: "existing-file"

src/common.h

@@ -810,11 +810,12 @@ void checkpoint_run (bool do_write);
 #define WARN_UNKNOWN_KEYWORD     0x00020000
 #define WARN_XDEV                0x00040000
 #define WARN_DECOMPRESS_PROGRAM  0x00080000
+#define WARN_EXISTING_FILE       0x00100000
 
 /* The warnings composing WARN_VERBOSE_WARNINGS are enabled by default
    in verbose mode */
 #define WARN_VERBOSE_WARNINGS    (WARN_RENAME_DIRECTORY|WARN_NEW_DIRECTORY|\
-				  WARN_DECOMPRESS_PROGRAM)
+				  WARN_DECOMPRESS_PROGRAM|WARN_EXISTING_FILE)
 #define WARN_ALL                 (~WARN_VERBOSE_WARNINGS)
 
 void set_warning_option (const char *arg);

src/extract.c

@@ -643,6 +643,8 @@ maybe_recoverable (char *file_name, bool regular, bool *interdir_made)
       switch (old_files_option)
 	{
 	case KEEP_OLD_FILES:
+	  WARNOPT (WARN_EXISTING_FILE,
+		   (0, 0, _("%s: skipping existing file"), file_name));
 	  return RECOVER_SKIP;
 
 	case KEEP_NEWER_FILES:

src/warning.c

@@ -42,6 +42,7 @@ static char const *const warning_args[] = {
   "unknown-keyword",
   "xdev",
   "decompress-program",
+  "existing-file",
   NULL
 };
 
@@ -66,7 +67,8 @@ static int warning_types[] = {
   WARN_UNKNOWN_CAST,
   WARN_UNKNOWN_KEYWORD,
   WARN_XDEV,
-  WARN_DECOMPRESS_PROGRAM
+  WARN_DECOMPRESS_PROGRAM,
+  WARN_EXISTING_FILE
 };
 
 ARGMATCH_VERIFY (warning_args, warning_types);