[patch] Split openFileForWrite into windows and unix versions

[Aaron Ballman]

One more question:

>> +  int FD = ::_open_osfhandle(intptr_t(H), 0);

If the caller wants to append, will this still cause appends to
happen?  Or should we specify _O_APPEND here too?  Which brings up a
similar question for the reading version of this API -- should we
specify _O_RDONLY for that one?

~Aaron