Removed the race condition in file_create() which could set a file to 0 if it

already exist.
I wanted to use O_EXCL, but `perldoc -f sysopen` doesn't say that O_EXCL is
avaiable in _every_ system. So I use O_RDONLY and O_CREAT.
Additionally, I replaced all the open() calls in this module by sysopen() calls.