Introduce quirks tunable for da driver

[bevan_bi-co.net]

Summary

Introduce a tunable kern.cam.da.X.quirks for devices handled by da, similar to that already existing for ada. This simplifies testing quirks for new devices as the kernel does not need to be recompiled.

I used this while trying to fix issues with a misbehaving drive. I don't know if this is something you may want to include, however I thought I'd post it here before it gets lost.

Test Plan

I tested this on FreeBSD 12.0-RELEASE and 13.0-CURRENT by adding the tunable to loader.conf, setting it to a combination of several quirks, and observing the kernel output when loading the corresponding device.

[mav]

Generally I think it is good, but I think it should be set higher, after the "softc->quirks |= DA_Q_NO_6_BYTE;" line.

[bevan_bi-co.net]

Thanks for the review! I agree it makes sense to move the code up so that other settings are derived accordingly.