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

1 files changed, 18 insertions, 0 deletions

diff --git a/t/lib/warnings/9enabled b/t/lib/warnings/9enabled
index a535689bad..68b0a27da3 100644
--- a/t/lib/warnings/9enabled
+++ b/t/lib/warnings/9enabled
@@ -1181,6 +1181,24 @@ my message 2 at - line 8
 my message 4 at - line 8
 ########
 
+--FILE-- abc52.pm
+package abc52 ;
+use warnings::register ('foo', 'bar');
+sub check {
+    warnings::warnif('abc52', "hello");
+    warnings::warnif('abc52::foo', "hello foo");
+    warnings::warnif('abc52::bar', "hello bar");
+}
+1;
+--FILE--
+use abc52;
+use warnings("abc52", "abc52::bar");
+abc52::check() ;
+EXPECT
+hello at - line 3
+hello bar at - line 3
+########
+
 --FILE--
 # test for bug [perl #15395]
 my ( $warn_cat, # warning category we'll try to control