[1.2.X] Fixed pk uniqueness validation for new objects created outside of a...

[1.2.X] Fixed pk uniqueness validation for new objects created outside of a ModelForm. Also removed need for ModelForm to poke at Model._state.adding, keeping it an internal ORM concern.

Backport of r14613.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14616 bcc190cf-cafb-0310-a4f2-bffc1f526a37