[analyzer] Fix taint rule of fgets and setproctitle_init

There was a typo in the rule.
`{{0}, ReturnValueIndex}` meant that the discrete index is `0` and the
variadic index is `-1`.
What we wanted instead is that both `0` and `-1` are in the discrete index
list.

Instead of this, we wanted to express that both `0` and the
`ReturnValueIndex` is in the discrete arg list.

The manual inspection revealed that `setproctitle_init` also suffered a
probably incomplete propagation rule.

Reviewed By: Szelethus, gamesh411

Differential Revision: https://reviews.llvm.org/D119129