Use command style in "help" command

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.