libminijail: pass-through errno should be negative
libminijail: pass-through errno should be negative

The errno values in the rest of libminijail use negative errno values. This
makes sure that the passed-through errno values are negative as well.

BUG=chromium-os:20903
TEST=Built for x86-alex and did a full image build & boot, ran okay as:
  sudo minijail0 -pu chronos /bin/ls
 and correctly failed (exit code 253) with:
  sudo minijail0 -S /dev/null /bin/ls

Change-Id: Ifac27468a21820ae342522c749c76f2045b630c3
Signed-off-by: Kees Cook <50cb10d2be062154b17ab4d66a4eab34f1cf1c84@chromium.org>
Reviewed-on: http://gerrit.chromium.org/gerrit/8394
Reviewed-by: Will Drewry <72416dde44ea762d19e3a7a555e3b59ce98f8eea@chromium.org>

omegaup/minijail,omegaup/minijail,omegaup/minijail,omegaup/minijail