Implemented multiseat support

[egormanga]

Abstract: tpm-fido can run multi-user, but any virtual device created by either instance would go to seat0 with no way to reassign it.

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loginctl assign requires ID_FOR_SEAT property to be set on the device, which is missing for hidraw devices due to ID_PATH_TAG not being set for those:

# /usr/lib/udev/rules.d/71-seat.rules
TAG=="seat", ENV{ID_FOR_SEAT}=="", ENV{ID_PATH_TAG}!="", ENV{ID_FOR_SEAT}="$env{SUBSYSTEM}-$env{ID_PATH_TAG}"

In order to add ID_PATH_TAG, we needed some way to distinguish between users' uhid devices. I came up with the following construction numbered lower than the aforementioned rule:

# /etc/udev/rules.d/70-tpm-fido.rules
SUBSYSTEM=="hidraw", KERNELS=="0003:15D9:0A37.*", IMPORT{parent}="HID_NAME"
SUBSYSTEM=="hidraw", ENV{HID_NAME}=="tpm-fido", ENV{ID_PATH_TAG}=="", ENV{ID_PATH_TAG}="$env{HID_NAME}-$attr{country}"

Clearly, I am using the country parent attribute here, but where does it come from? See psanford/uhid#2 to find out ;)

Note: I chose to use UID for the value, which gets truncated to 8 bits, but that's diverse enough for most multiseat configuration out in the wild.