When more than one printer is connected, ptouch-print used the first
one it found. --serial <serial> now uses only the printer with that USB
serial number, and --list-connected lists the connected printers with
their serials.
libptouch gets ptouch_open_serial() and ptouch_list_connected();
ptouch_open() keeps its signature and calls ptouch_open_serial() with
NULL. The serial is checked before the P-Lite and unsupported-raster
checks, so a different printer in the wrong mode no longer ends the
search, and the "found on USB bus" line now shows the serial. The early
error returns in the open loop now free the device list.
devinfo is malloc'd and min_timeout was never set, so the status
timeout was whatever happened to be in memory (seen as
"timeout (1086334400 sec) while waiting for status response").
James A