The keyd package already exists, but without a systemd service.
Keyd requires write access to /var/run to create its socket. Currently
the directory it uses can be changed with an environment variable, but
the keyd repo state suggests that this may turn into a compile-time
option. with that set, and some supplementary groups added, we can run
the service under DynamicUser.
Co-authored-by: pennae <82953136+pennae@users.noreply.github.com>
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