daemon: un-export IsRunningSystemd()

This utility was added after 19.03, and is only used in the daemon code
itself, so we can un-export it, until there's an external use for it.

Also updated the description, because the runc code already copied it
from coreos/go-systemd, so better to describe the actual source.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/daemon_unix.go

@@ -638,14 +638,20 @@ func UsingSystemd(config *config.Config) bool {
 		return true
 	}
 	// On cgroup v2 hosts, default to systemd driver
-	if getCD(config) == "" && cgroups.Mode() == cgroups.Unified && IsRunningSystemd() {
+	if getCD(config) == "" && cgroups.Mode() == cgroups.Unified && isRunningSystemd() {
 		return true
 	}
 	return false
 }
 
-// IsRunningSystemd is from https://github.com/opencontainers/runc/blob/46be7b612e2533c494e6a251111de46d8e286ed5/libcontainer/cgroups/systemd/common.go#L27-L33
-func IsRunningSystemd() bool {
+// isRunningSystemd checks whether the host was booted with systemd as its init
+// system. This functions similarly to systemd's `sd_booted(3)`: internally, it
+// checks whether /run/systemd/system/ exists and is a directory.
+// http://www.freedesktop.org/software/systemd/man/sd_booted.html
+//
+// NOTE: This function comes from package github.com/coreos/go-systemd/util
+// It was borrowed here to avoid a dependency on cgo.
+func isRunningSystemd() bool {
 	fi, err := os.Lstat("/run/systemd/system")
 	if err != nil {
 		return false