More control for isa_plugin() arguments

When a plugin is wrapped with an isa_plugin() call, the plugin we're
inheriting from will receive the same arguments as the wrapper. This
patch allows to modify the argument list for the wrapped plugin, i.e.
 * delete arguments only valid for the wrapping plugin
 * add additional arguments
Modifying @_ directly only allows something like "for (@_) { s/a/b/g }",
not adding or deleting. Returning the new @_ will much likely break
existing plugins.

docs/plugins.pod

@@ -176,10 +176,18 @@ All valid hooks are defined in F<lib/Qpsmtpd/Plugins.pm>, C<our @hooks>.
 =head2 Inheritance
 
 Inheriting methods from other plugins is an advanced topic. You can alter 
-arguments for the underlying plugin, prepare something for the I<real> 
-plugin or skip a hook with this. Instead of modifying C<@ISA>
+arguments for the underlying plugin (see below), prepare something for the
+C<real> plugin or skip a hook with this. Instead of modifying C<@ISA>
 directly in your plugin, use the C<isa_plugin()> method from the 
-C<init()> subroutine.
+C<init()> subroutine. 
+
+Altering arguments for the C<real> plugin is done by calling 
+C<$self-E<gt>set_isa_plugin_args(@args)> from C<init()>. If you do not 
+call this, the C<real> plugin will receive the same arguments as your 
+plugin (modulo changes done with C<for(@_) { tr/a-z/A-Z/; }> or similar,
+see L<perlsub(1)>): C<init()> and C<register()> are called with the same 
+C<@_> as argument. Deleting or adding plugin arguments B<must> be done with 
+C<set_isa_plugin_args()>.
 
   # rcpt_ok_child
   sub init {

lib/Qpsmtpd/Plugin.pm

@@ -45,13 +45,27 @@ sub register_hook {
     );
 }
 
+my @isa_plugin_args = ();
+sub set_isa_plugin_args {
+  my $self = shift;
+  $self->{_isa_plugin_args} = 1;
+  @isa_plugin_args = @_;
+}
+
 sub _register {
   my $self = shift;
   my $qp = shift;
   local $self->{_qp} = $qp;
+
+  @isa_plugin_args = ();
   $self->init($qp, @_)     if $self->can('init');
+  $self->{_isa_plugin_args} 
+    and @_ = @isa_plugin_args;
+
   $self->_register_standard_hooks($qp, @_);
   $self->register($qp, @_) if $self->can('register');
+
+  delete $self->{_isa_plugin_args};
 }
 
 sub qp {