Fixed #16715 -- Fixed join promotion logic for nested nullable FKs

The joins for nested nullable foreign keys were often created as INNER
when they should have been OUTER joins. The reason was that only the
first join in the chain was promoted correctly. There were also issues
with select_related etc.

The basic structure for this problem was:
  A -[nullable]-> B -[nonnull]-> C

And the basic problem was that the A->B join was correctly LOUTER,
the B->C join not.

The major change taken in this patch is that now if we promote a join
A->B, we will automatically promote joins B->X for all X in the query.
Also, we now make sure there aren't ever join chains like:
   a LOUTER b INNER c
If the a -> b needs to be LOUTER, then the INNER at the end of the
chain will cancel the LOUTER join and we have a broken query.

Sebastian reported this problem and did also major portions of the
patch.

tests/regressiontests/nested_foreign_keys/models.py

@@ -0,0 +1,28 @@
+from django.db import models
+
+
+class Person(models.Model):
+    name = models.CharField(max_length=200)
+
+
+class Movie(models.Model):
+    title = models.CharField(max_length=200)
+    director = models.ForeignKey(Person)
+
+
+class Event(models.Model):
+    pass
+
+
+class Screening(Event):
+    movie = models.ForeignKey(Movie)
+
+class ScreeningNullFK(Event):
+    movie = models.ForeignKey(Movie, null=True)
+
+
+class Package(models.Model):
+    screening = models.ForeignKey(Screening, null=True)
+
+class PackageNullFK(models.Model):
+    screening = models.ForeignKey(ScreeningNullFK, null=True)