improve registration of warning categories

improve registration of warning categories

1. &warnings::register is added as the public mechanism for adding
   new warning categories, rather than warnings::register::import
   knowing about warnings's internals

2. warnings::register::import is updated to use &warnings::register

3. warnings::register::import can take a list of subcategories

The upshot is that you can now write:

  package MyTool;
  use warnings::register qw(io typos);

  warnings::warnif('MyTool::io', $message);

...and tools that register new warnings categories do not need to cargo cult
code from warnings/register.pm