Avoid Boost.Filesystem: Boost package improperly built on Windows?

Seems Linden's Boost package and the viewer build might use different
settings of the /Zc:wchar_t switch.
Anyway, this implementation using open(O_CREAT | O_EXCL) should be more
robust. I'm surprised Boost.Filesystem doesn't seem to offer "create a unique
file"; all I found was "generate a random filename fairly likely to be unique."