I broke this in !321

This would allow logd to write all received log messages to debug:
without causing an infinite loop when it at the same time reads all
messages in sys:log.

Diagonal as in

Thread A        Thread B
Send to B       Send to A
Wait for B      Wait for A.

I haven't reproduced this bug, but this should fix it, since the wait
steps now checks the local percpu block for pending TLB shootdowns onto
itself.

Rather than disabling it right before entering userspace. This allows
showing debug messages while the graphics subsystem hasn't initialized
yet.

This doesn't give any issues both for UEFI boot (with framebuffer
present) and U-Boot boot (without framebuffer present).

This fixes a bug where the window of `viewer` does not exit, due to a
leaked file descriptor.