unshare,nsenter: spawn shell by default

unshare,nsenter: spawn shell by default

The behaviour mimics chroot.

Possibly it would have been nicer to to query the password database in
the new namepace and run the shell of the user there, but it's hard to
do correctly. getpwuid() might need to load nss plugins, and the arch
in the new namespace might be different (in case of NEWNS mounts), or
the hostname might be different, etc. So in general it's not possible
to do it reliably.

Signed-off-by: Zbigniew Jędrzejewski-Szmek <zbyszek@in.waw.pl>