fix airprint discovery

cupsd.conf

@@ -20,6 +20,18 @@ Browsing On
 BrowseLocalProtocols dnssd
 BrowseWebIF Yes
 
+# Do not register the CUPS "_cups" Bonjour sub-type.
+#
+# macOS decides between the driverless AirPrint driver and its generic
+# PostScript driver in its Add Printer browser (PMBrowser.framework): it offers
+# AirPrint only for a service carrying the "_universal" sub-type, and it treats
+# a service as *shared* as soon as it also carries "_cups"
+# (-[DNSSDPrinter isShared]; -[DNSSDPrinter airprintMatch] then returns
+# nothing). CUPS registers "_cups,_print,_universal" by default, which is why
+# the queue is offered as "Generic PostScript Printer" only. Keep "_print" for
+# clients that look for it.
+BrowseDNSSDSubTypes _print,_universal
+
 # Default authentication type, when authentication is required...
 DefaultAuthType Basic