fstatfs() on the open file to verify that no intruder has mounted
something on portions of /proc. This will catch, for instance,
"mount_null /tmp /proc/1378". We already do 5 syscalls per
process, so one more won't hurt :) ... and safety is better than
performance when ps is otherwise broken.
- Also added a few close()'s at early returns, to avoid chewing up fd's.
2721f671ab