Use command style in "help" command

This changes the help command to use the new command style when
displaying text like:

    List of "catch" subcommands:

As a side effect, this mildly -- but not hugely -- cleans up some i18n
issues in help_list.  The header comment for that function is also
changed to the gdb style.

Finally, this function used to print something like:

    Type "help catch" followed by catch subcommand name for full documentation.

The second "catch" here seems redundant to me, so this patch removes
it.

gdb/cli/cli-decode.h

@@ -83,6 +83,9 @@ struct cmd_list_element
      For non-prefix commands, return an empty string.  */
   std::string prefixname () const;
 
+  /* Like prefixname, but do not append a trailing space.  */
+  std::string prefixname_no_space () const;
+
   /* Return a vector of strings describing the components of the full name
      of this command.  For example, if this command is 'set AA BB CC',
      then the vector will contain 4 elements 'set', 'AA', 'BB', and 'CC'