Fix delays printing to lpd when reserved ports are exhausted

Fix delays printing to lpd when reserved ports are exhausted

cups_rresvport() doesn't reserve ports less than 512; however,
lpd_queue() continues decrementing the port number to 0.  This leads
to delays of ~511 seconds once all ports between 512-1023 are
exhausted.  Even when ports become available, lpd_queue() still tries
calling cups_rresvport() with port numbers less than 512, waiting one
second between each call.