Fixed #22502 -- Fixed microseconds/default/form interaction

Made explicit lack of microsecond handling by built-in datetime form
fields. Used that explicitness to appropriately nix microsecond
values in bound fields. Thanks Claude Paroz for the review.
Adjusted refactoring of vendor checks.

Thanks Shai for the thorough review.
Fixed #22508 -- Avoided overwriting select_related.

Previously, known related objects overwrote related objects loaded
though select_related. This could cancel the effect of select_related
when it was used over more than one level.

Thanks boxm for the bug report and timo for bisecting the regression.
Minor cleanup in select_related_regress tests.
Fixed #21685 -- Displayed app verbose name in admindocs model index