p910nd: Run as non-root by default

We add the necessary Makefile, hotplug, config, and init bits
so that p910nd daemon runs as user:group p910nd:lp by default.
This eliminates an unnecessary root daemon.

The hotplug script sets the permissions of the USB lp
device(s) to read-write owner and group and no access to
anyone else, and sets owner root, group lp.

This is allows sufficient privileges to p910nd
to do it's job.

(cherry-pick 932c76fa74)

Signed-off-by: Daniel F. Dickinson <cshored@thecshore.com>

net/p910nd/files/p910nd.init

@@ -28,7 +28,7 @@ start_service() {
 
 
 start_p910nd() {
-	local section="$1"
+	local section="$1" runas_root
 	config_get_bool "enabled" "$section" "enabled" '1'
 	if [ "$enabled" -gt 0 ]; then
 		args="-d "
@@ -42,6 +42,9 @@ start_p910nd() {
 		procd_set_param command /usr/sbin/p910nd $args
 		procd_set_param respawn
 
+		config_get_bool runas_root "$section" runas_root 0
+		[ "$runas_root" -ne 1 ] && procd_set_param user p910nd
+
 		config_get_bool "mdns" "$section" "mdns" '0'
 		config_get mdns_note "$section" mdns_note
 		config_get mdns_ty "$section" mdns_ty