ffiselect: Set /proc/../oom_score_adj

We should be setting /proc/[childpid]/oom_score_adj to a high value like 900 on tasks which aren't PoSt so that:

• The main process is less likely to be killed
• Other processes on the machine are less likely to be killed (Lotus, yugabyte)
• Things don't break horribly when the scheduler misjudges how much memory we have (i.e. something outside of curio uses a bunch of memory in the middle of a memory-heavy task)