Fixed bug with disable and enable leading to mask problems

modifyPrincipal now sets the mask to 0 if it is not given a mask, rather
than passing along the undefined mask.  This directly affected the
disablePrincipal and enablePrincipal functions, which would call
modifyPrincipal without a given mask and led to problems on my system.

Kadm5.pm

@@ -313,6 +313,7 @@ sub modifyPrincipal
   {
     my ($self,$principal,$mask) = @_;
     
+    $mask = 0 unless $mask;
     eval
       {
 	$self->handle->c_modify_principal($principal,$mask);