Have builtin use namespace magic

builtin will now grab all the files in /src/cmds/sh and the created builtin.go,
privatize the namespace, mount a tmpfs on /bin and /src/cmds/sh, drop the .go files
in /src/cmds/sh, and exec /buildbin/sh.

This nicely isolates the changes from the calling process.

Note that with real union mounts, as we might have them in 3.18, this process can be
further simplified. But we'll do that when we're ready.

Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>