There were two problems here:
* This would fail when run with unlimited stack, because it didn't know
that bionic reports unlimited stacks as 8MiB.
* This would leave RLIMIT_STACK small, causing failures to exec (so the
popen and system tests would fail).
(cherry-pick of 27a9aed81978af792cb06035a1619c8141a5fb5b plus the
new ScopeGuard.h from a3ad450a2e3fb6b3fe359683b247eba20896f646.)
Bug: 17394276
Change-Id: I5b92dc64ca089400223b2d9a3743e9b9d57c1bc2