gdb: make copyright.py interface a bit nicer

This way people can run `./copyright.py --help` and get some info as
to what this does without it going and modifying the tree.

gdb/copyright.py

@@ -22,7 +22,7 @@
 This script updates the list of years in the copyright notices in
 most files maintained by the GDB project.
 
-Usage: cd src/gdb && python copyright.py
+Usage: cd src/gdb && ./copyright.py
 
 Always review the output of this script before committing it!
 A useful command to review the output is:
@@ -30,12 +30,14 @@ A useful command to review the output is:
 This removes the bulk of the changes which are most likely to be correct.
 """
 
+import argparse
 import datetime
 import locale
 import os
 import os.path
 import subprocess
 import sys
+from typing import List, Optional
 
 
 def get_update_list():
@@ -158,16 +160,25 @@ def may_have_copyright_notice(filename):
     return False
 
 
-def main():
+def get_parser() -> argparse.ArgumentParser:
+    """Get a command line parser."""
+    parser = argparse.ArgumentParser(
+        description=__doc__, formatter_class=argparse.RawDescriptionHelpFormatter
+    )
+    return parser
+
+
+def main(argv: List[str]) -> Optional[int]:
     """The main subprogram."""
+    parser = get_parser()
+    _ = parser.parse_args(argv)
     root_dir = os.path.dirname(os.getcwd())
     os.chdir(root_dir)
 
     if not (
         os.path.isdir("gdb") and os.path.isfile("gnulib/import/extra/update-copyright")
     ):
-        print("Error: This script must be called from the gdb directory.")
+        sys.exit("Error: This script must be called from the gdb directory.")
-        sys.exit(1)
 
     update_list = get_update_list()
     update_files(update_list)
@@ -416,4 +427,4 @@ NOT_FSF_LIST = (
 )
 
 if __name__ == "__main__":
-    main()
+    sys.exit(main(sys.argv[1:]))