Use "NO NAME" as the default unnamed label.

Microsoft recommends avoiding the use of spaces in the
string structures for FAT. Unfortunately they do just
that by default in the case of unlabeled filesystems.

Follow the default MS behavior to avoid confusion in
common tools like file(1). This was actually the
default behavior before r203868.

Obtained from:	NetBSD (CVS rev. 1.39)
MFC after:	3 days