stenc should detect an empty LTO drive and return a proper exit code

[devnullzero]

When running stenc on a drive with not cartridge, it "waits" doing nothing.

Expected behavior:
stenc should detect an empty drive and return a proper exit code.

[scsirob]

The SCSIExecute() routine in scsiencrypt.cpp blocks when opening a device that is not ready. When the device is not ready, the Linux kernel keeps polling the drive with SCSI Test Unit Ready commands.

Change the line
sg_fd = open(tapedevice, O_RDONLY);

to
sg_fd = open(tapedevice, O_RDONLY|O_NDELAY);