fileio: add WRITE_STRING_FILE_MODE_0600 flag for writing files

fileio: add WRITE_STRING_FILE_MODE_0600 flag for writing files

usually we want to create new files with mode 0666 (modulated by the
umask). Sometimes we want more restrictive access though, let's add an
explicit flag support for that.

(Note that we don't bother with arbitrary access modes to keep things
simple: just "open as umask permits" and "private to me", nothing else)