routine sets the process's file mode creation mask to
and returns the previous value of the mask.
The 9 low-order
are used by system calls, including
to turn off corresponding bits
requested in file mode.
This clearing allows each user to restrict the default access
to his files.
The default mask value is S_IWGRP|S_IWOTH (022, write access for the
Child processes inherit the mask of the calling process.