printers: Add functions for searching for LPD printers

Add pp_host_get_lpd_devices_async() and
pp_host_get_lpd_devices_finish() functions to PpHost class.
pp_host_get_lpd_devices_async() starts searching for LPD printer
on given address.
The test consist in connection to the default port 515 (or the one
given by creator of PpHost) of the address and sending a print job
to it, which in turn returns a status (we test several standard
queue names).
We suppose that there is a LPD printer on the address if a buffer
with zero length is returned (rfc 1179 5.2).

https://bugzilla.gnome.org/show_bug.cgi?id=695564

panels/printers/pp-utils.h

@@ -54,7 +54,8 @@ enum
   ACQUISITION_METHOD_SNMP,
   ACQUISITION_METHOD_SAMBA,
   ACQUISITION_METHOD_SAMBA_HOST,
-  ACQUISITION_METHOD_JETDIRECT
+  ACQUISITION_METHOD_JETDIRECT,
+  ACQUISITION_METHOD_LPD
 };
 
 typedef struct