Do media checks on getting next block encryption status

[jmwilson]

Only the next block encryption status query (aka "Volume Encryption" in the output) requires having media loaded, so allow stenc to run on a drive without media by opening the device in nonblocking mode, and checking for media before getting NBES.

Additionally, as reported in #71, it appears some drives may return BLANK CHECK status on unformatted media instead of GOOD status with "cannot determine" in the result. Catch this specific error and suppress the error message.

Closes #12 and #71.

[sunwire]

Before we close #12 What about FreeBSD? Does it also need O_NDELAY flag?

stenc/src/scsiencrypt.cpp

Lines 113 to 115 in 8caaf85

	#elif defined(OS_FREEBSD)
	auto dev = std::unique_ptr<struct cam_device, decltype(&cam_close_device)>
	{cam_open_device(device.c_str(), O_RDWR), &cam_close_device};

[jmwilson]

No need; that code finds the generic device and opens it, which doesn't do any media check. So this makes the two versions more consistent.