Add os::exists(pid_t pid) to test if pid is running.

Uses kill(0, pid) to check pid validity. This is much cheaper than using
os::process(pid) which constructs a full Process object.

Review: https://reviews.apache.org/r/20970
Update process::reap to use os::exists(pid).

Review: https://reviews.apache.org/r/20971
Move OOM log message to appear only on actual OOM

Review: https://reviews.apache.org/r/20139