S9 EXT  (sys:umask integer)  ==>  integer
        (sys:umask)          ==>  integer

When applied to a single integer argument, the SYS:UMASK procedure
sets the file creation mode mask of the S9 process and returns the
old umask value. When called without any argument, it simply returns
the current umask value.

The new file creation mode mask will be the lower nine bits of INTEGER,
corresponding to the following file access permission bits:

        User    Group   Other
        r w x   r w x   r w x
        8 7 6   5 4 3   2 1 0

When creating a file with the SYS:CREAT procedure or the
OPEN-OUTPUT-FILE function, the bits set in the file creation
mask will be zeroed in the permission bits of the new file.
E.g., the default mask of #o022 will never allow the group
or others to write to the new file.

(begin (sys:umask #o22) (sys:umask))  ==>  18
