Fix breakage introduced by #887

[squell]

sudo-rs was actually broken by #887, because that changeset introduced a bug in the /proc/stat parser.

This PR undoes that change (in particular, it undoes e33d640).

I could have tried to keep the 1-based indexing, but:

• I wasn't broken.
• I actually prefer 0-based indexing

This also adds a simple unit test for read_proc_stat that would have caught this error.

[bjorn3]

Oops, the correct thing to do would have been to increment the curr_field initializer from 1 to 2 I think, but I agree that just reverting the change is a better option.