[racket] call-with-timeout

FWIW, I ended up using "call-with-limits".  (After I added the missing 
"kill-thread" to my "call-with-timeout", I couldn't tell whether 
"call-with-timeout" or "call-with-limits" was better, so I just went 
with the stock thing.)

BTW, from looking at the code, "call-with-limits" does change the 
current custodian, so people using "call-with-limits" might need some 
understanding of custodians.

Neil V.