Allow reading the jailed process' stdout and stderr.

Also fix some nits while in there.

BUG=None
TEST=libminijail_unittest on alex and lumpy.

Change-Id: I1bd227f196618d275da6e5da4ce91e90a370baa2
Reviewed-on: https://gerrit.chromium.org/gerrit/43460
Reviewed-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Tested-by: Jorge Lucangeli Obes <jorgelo@chromium.org>
Commit-Queue: Jorge Lucangeli Obes <jorgelo@chromium.org>
3 files changed