do not exit with an error when $command --help is successfully used

When checking for a minimum requirement of arguments and aborting with
the help text otherwise, we should first check to see if the help text
was explicitly asked for.

src/calibre/db/cli/main.py

@@ -238,6 +238,9 @@ def main(args=sys.argv):
     if len(args) < 2:
         parser.print_help()
         return 1
+    if args[1] in ('-h', '--help'):
+        parser.print_help()
+        return 0
     if args[1] == '--version':
         parser.print_version()
         return 0

src/calibre/ebooks/conversion/cli.py

@@ -308,7 +308,10 @@ def create_option_parser(args, log):
     parser = option_parser()
     if len(args) < 3:
         print_help(parser, log)
-        raise SystemExit(1)
+        if any(x in args for x in ('-h', '--help')):
+            raise SystemExit(0)
+        else:
+            raise SystemExit(1)
 
     input, output = check_command_line_options(parser, args, log)