OS.FileSys.tmpName should create a temporary file, not just name it about mosml

[Munksgaard]

A small comment on the basis library specification: It actually says that the file should be inaccessible to other users. One obvious way to do this would be to create the temporary file, and return some kind of file handle to the file instead of the file itself. However, we are required to return the filename itself, but creating the temporary file and returning the filename while keeping the file handle open wont work.

SML/NJ uses mkstemp, but closes the resulting file handle and returns the filename. In effect, the security issues that were the reason for deprecating tmpnam are still present; other processes can potentially access the temporary file in between the call to tmpName and actually opening the file.