Fixes #5140: visudo on Debian 6 cannot use strict mode

I don't understand. This code implies that only version 1.7.4 of sudo didn't support the  -s flag. Is that really true? If so why? This definitely warrants an explanation in a comment on the code, preferably with a link to an online bug report or similar explaining this. 

If it's really "<= 1.7.4", then please adapt the code to actually implement that.