vt(4): Use power_{suspend,resume} event handlers to implement suspend/resume

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Summary

The goal is to avoid that the vt(4) resume happens before the video display is resumed. The original patch was provided by Andriy Gapon.

This new patch registers the handlers in vt_upgrade(). This is done once, thanks to the VDF_ASYNC flag. I abused this flag because it was already abused by the keyboard allocation. The event handlers then call the backend if it provides callbacks for suspend/resume.

[avg]

I've also came to conclusion that for vt switching to do its intended job reliably vt_suspend() should not only request the switch but also wait until it is completed. Waiting was scary to do from the bus suspend context, but should be okay in the power_suspend context.

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A patch against the current code (not on top of this reviewed patch): https://github.com/avg-I/freebsd/commit/cec4e6e806213d622d88db3745fe9c8194a1a621

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Changes:

• Wait for window switch in the suspend path (patch from Andriy Gapon)

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Changes:

• Try to remove modification to kern_cons.c

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Was someone able to test this patch? My computer doesn't suspend/resume, so can't do it myself.

[avg]

The change looks good to me and, as far as my testing goes, it works as well.

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The change was committed in rS277795.