Check usability of named device following a non-existent / invalid one (#756434)

When multiple devices are named on the command line and (after sorting
and removing duplicates) the device following a non-existent or invalid
one is not checked for usability [1].  In most situations this isn't
noticed as the device gets skipped at the "Searching ...  partitions"
step instead.  However as seen in bug 755495 and commit [2]
checking usability matters.

For example (on CentOS 6.5) a large sector disk device can be edited
when it follows a non-existent or invalid device named on the command
line:

    # modprobe scsi_debug dev_size_mb=128 sector_size=4096
    # fgrep scsi_debug /sys/block/*/device/model
    /sys/block/sdd/device/model:scsi_debug

    # ./gpartedbin /dev/does-not-exist /dev/sdd
    ======================
    libparted : 2.1
    ======================
    Could not stat device /dev/does-not-exist - No such file or directory.
    Device /dev/sdd has a logical sector size of 4096.  Not all parts of ...