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.config

@@ -5,6 +5,8 @@ config p910nd
 	option port          0
 	option bidirectional 1
 	option enabled       0
+	# Override running as user p910nd, group lp
+	option runas_root    0
 
 	# mDNS support - see Bonjour Printing Specification for details concerning the values
 	# Be aware that you can only advertise one printer on this host via mDNS