unshare: load binfmt_misc interpreter

unshare: load binfmt_misc interpreter

add -l, --load-interp <file> to load a binfmt_misc interpreter at startup.

The interpreter is loaded from the initial fileystem if the 'F' flags is
provided, otherwise from inside the new namespace
This makes possible to start a chroot of another architecture without
being root.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Gbp-Pq: Topic upstream-master
Gbp-Pq: Name unshare-load-binfmt_misc-interpreter.patch