Do not inherit host environment when entering the Sandstorm outer container.

Many kinds of environment settings become invalid or problematic after a chroot (or pivot_root in our case). For instance, an LD_PRELOAD would now point at a directory that doesn't exist. More commonly, a LANG setting other than C or C.UTF-8 could be problematic as the locale files distributed it the Sandstorm bundle is taken from the machine where the bundle was built and probably don't support arbitrary languages.
Fix id(1) output parsing in non-English locales. Fixes #122.