argv0 runs a program with a specified 0th argument.
If you don't know what this is, you probably don't need it. This is essentially the same as djb's argv0 program, except that you don't need to build all of ucspi-tcp to use it. (And I only found out that djb wrote it after I did, of course. In fact, I flipped around the argument order to be compatible with his, so there.)