Do not attempt seccomp under qemu-user and drop EFAULT workaround

qemu-user passes prctl()-based seccomp through to the kernel,
umodified. That's bad, as it blocks the wrong syscalls.

We ignored EFAULT which fixed the problem for targets with different
pointer sizes from the host, but was a bad hack. In order to identify
qemu we can rely on the fact that qemu-user prints its version and
exits with 0 if QEMU_VERSION is set to an unsupported value. If we
run a command that should fail in such an environment, and it exits
with 0, then we are running in qemu-user.

apt-helper is an obvious command to run. The tests ensure it exits
with 1, and it only prints usage information. We also could not use
/bin/false because apt might just as well be from a foreign arch
while /bin/false is not.

Closes: #881519

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