In preparation for seccomp execve inheritance.
We are going to need to add a new environment variable earlier in the
execve sequence to handle inheritance in the case of binfmt_misc.
No functional change in regards to envp handling.
Minor change around execveat with FD without binfmt_misc. In the case
that execveat returned an error and we did a `dup` of the FD then we
would have an FD leak. Make sure to close the duplicated FD in that
instance.