Add oom_score_adj Process methods

[futpib]

I took note of this comment: #139 (review):

So the way I read this is: while the final OOM "score" will be somewhere between 0 and 1000, the value reported in oom_score_adj can be between -1000 and 1000. So I think the oom_score_adj function should return a i16 not u32.

And changed oom_score type to u16 too, since it too only ranges from 0 to 1000.

Previous attempt: #139

[eminence]

LGTM, thanks!