[racket] Setting umask from within Racket

If you don't find exactly the solution you want, possible measures:

* If the concern is security, create the files in a directory on which 
you've set sufficiently restrictive permissions.  (Even if you always 
set the file permissions immediately after opening the file for writing, 
there could be a race condition that lets the file be opened between 
create and permissions set.)

* If you are ruthless, you could set the umask for entire process, using 
the FFI for the system call.

* If you don't want to FFI, but you really-really want to second-guess 
the parent process's umask, you could kludge it in pure Racket plus host 
processes, at least on Unix-ish systems.  Have the Racket program check 
its umask, and if not the desired one, use process/shell API to create a 
new process of the same Racket program, with the same command line, but 
under the new umask.

* Consider writing your own pure Racket convenience procedures for 
opening files with desired permissions.  (Maybe you could even have it 
secretly do tricks to do the non-atomic create and chmod securely 
without messing with FFI or umask.  Some tricks of moving and copying 
files might occasionally run afoul of unusual situations in the 
filesystem, and of finer points of different filesystem semantics, but 
in practice, most people will never encounter any problems with simple 
tricks.)

Side note: The Unix o/g/a-r/w/x permissions are pragmatic and archaic, 
so (like much of Unix) don't read too much into them, but they seem to 
have survived OK.  And there seems to be less need for fancier ACLs and 
compartmentalization and such, now that we've moved most multi-user 
access control out of the filesystem and into Internet protocols and 
larger structures of same.  And into VMs.

Neil V.