update documentation to explain systemd socket-activation configuration

doc/kresd.8.in

@@ -111,6 +111,10 @@ With this option, the daemon is started in non-interactive mode and instead crea
 UNIX socket in \fIrundir\fR that the operator can connect to for interactive session.
 A number greater than 1 forks the daemon N times, all forks will bind to same addresses
 and the kernel will load-balance between them on Linux with \fISO_REUSEPORT\fR support.
+
+When socket-activated and supervised by systemd or the equivalent, kresd defaults to
+--forks=1, and must not be set to any other value.  If you want multiple concurrent
+processes supervised in this way, they should be supervised independently.
 .TP
 .B \-q\fR, \fB\-\-quiet
 Daemon will refrain from printing any informative messages, not even a prompt.

scripts/kresd.service

-[Unit]
-Description=Knot DNS Resolver daemon
-After=network.target
-
-[Service]
-Type=simple
-EnvironmentFile=-/etc/sysconfig/kresd
-ExecStart=/usr/sbin/kresd -c /etc/kresd/config -f $KRESD_WORKERS $KRESD_OPTIONS /var/lib/kresd/
-Restart=on-abort
-
-[Install]
-WantedBy=multi-user.target

scripts/kresd.sysconfig

-## Path:        System/DNS
-## Description: Number of worker processes to spawn
-## Type:        integer
-## Default:     1
-## ServiceRestart:      kresd
-##
-#
-# Number of workers to spawn for kresd.
-# If you get start up failures with "already in use" your libuv is too
-# old and you have to stick to 1.
-#
-KRESD_WORKERS=1
-# Additional options
-KRESD_OPTIONS=

systemd/README.md

+Running Knot Resolver under systemd (or equivalent) socket activation
+=====================================================================
+
+You can use the files in this directory to run kresd under supervision
+by systemd (or any supervisor that provides equivalent file descriptor
+initialization via the interface supported by
+sd_listen_fds_with_names(3)).
+
+When run in this configuration:
+
+ * it will be run under a non-privileged user, which means it will not
+   be able to open any new non-privileged ports.
+
+ * it will use a single process (implicitly uses --forks=1, and will
+   fail if that configuration variable is set to a different value).
+   If you want multiple daemons to listen on these ports publicly
+   concurrently, you'll need the supervisor to manage them
+   differently, for example via a systemd generator:
+
+     https://www.freedesktop.org/software/systemd/man/systemd.generator.html
+
+   If you have a useful systemd generator for multiple concurrent
+   processes, please contribute it upstream!