Fixes <https://issues.guix.gnu.org/47681>.
This change adjusts the location of the udev configuration file and rules
directory to a fixed location. Since udev relies on inotify to discover
change to its rules directory (/etc/udev/rules.d), by using a fixed directory
layout, new udev rules can be automatically picked up without restarting the
service.
* gnu/services/base.scm (udev-rules-union): Build rules output directly
in #$output.
(udev-shepherd-service)[start]: Adjust the UDEV_CONFIG_FILE and
EUDEV_RULES_DIRECTORY environment variables.
[actions]: Remove field. The 'rules' action is no longer useful.
(udev.conf): New variable.
(udev-etc): New procedure.
(udev-service-type): Extend the etc-service-type with it.
e9fa17eb98