Boot failures with 14.0-ALPHA1

I'll update to ALPHA2.

In the meantime, briefly: <https://photos.app.goo.gl/jcXxDuxWPyXTa59Z8>

– no visible progress beyond the four lines of EFI framebuffer information.

In each case, I worked around by removing a USB flash drive that's used 
for L2ARC.

The most recent incident was preceded by a power button shutdown, in 
response to a drm-510-kmod blackout (GPU lockup etc.) whilst typing in 
Firefox (comparable to <https://github.com/freebsd/drm-kmod/issues/239> 
for drm-515-kmod 5.15.25).

The prior incident was also preceded by a power button shutdown (in 
response to a blackout at Plasma log out time). This was remarkable in 
that there was an unexpected restart, when I expected the computer to 
power off.