[1.6.x] Fixed #20988 -- Added model meta option select_on_save

The option can be used to force pre 1.6 style SELECT on save behaviour.
This is needed in case the database returns zero updated rows even if
there is a matching row in the DB. One such case is PostgreSQL update
trigger that returns NULL.

Reviewed by Tim Graham.

Refs #16649

Backport of e973ee6a9879969b8ae05bb7ff681172cc5386a5 from master

Conflicts:
	django/db/models/options.py
	tests/basic/tests.py