bug#47681: Reloading udev rules requires a system restart

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hello Guix!

Using Guix System, after adding a new rule to the configuration of their
udev-service-type service, the only ways to get the new rule into effect
are to either:

1. restart udev

(which is almost the same as a reboot, bringing down your graphical session)

2. restart the operating system

Both of which are sub-optimal.

This is caused by the configuration file/rules being made known to udev
via environment variables:

$ sudo cat /proc/$(pgrep udev)/environ | xargs -0 -n1 echo
UDEV_CONFIG_FILE=/gnu/store/7yfpf8acjy884xbwaq5kn9z21irchfaj-udev.conf
EUDEV_RULES_DIRECTORY=/gnu/store/yv58b7rg7dm3191cj6sma896550wgy4v-udev-rules/lib/udev/rules.d
LINUX_MODULE_DIRECTORY=/run/booted-system/kernel/lib/modules
PATH=/run/current-system/profile/bin

For convenience, we should probably have the udev-service-type create a
union of what it needs under /etc/udev/ as on other distributions.  udev
could then rely on a fixed location to look things and use its inotify
based mechanism to detect changes and reload automatically when needed.

This could probably fix things such as 'udevadm test' only reading rule
files from under
/gnu/store/svplp9wl0g2ahlv5rf6bhmq3xvp4zzh3-eudev-3.2.9/lib/udev/rules.d,
for example.

Thank you,

Maxim