Bug 912820: Make --disable-content-sandbox* configure flags work. r=gps
Bug 912807: Enable sandbox reporter by default on b2g.

This is a workaround for bug 907006, but also allows people who trip
over false positives in our sandbox to find out why.
Bug 908907: Fill in gaps in seccomp-bpf whitelist for b2g.

Relatively harmless syscalls:
* dup, used by mozilla::ipc::Shmem
* getuid, for android::IPCThreadState, used in audio decode
* nanosleep, used by android::AudioTrack

Of potential concern:
* sched_setscheduler, used by audio threads in e.g. CubeVid
  This might be restrictable somewhat by inspecting its arguments.

Of serious concern:
* unlink, as a workaround for bug 906996 (q.v.).

Note that we already allow open(), including for writing (temporary
files, /dev/genlock on qcom devices, probably more), so allowing unlink
won't make the situation much worse.