Use unshare() instead of clone()

It turns out that using clone() to start a child process is unsafe in
a multithreaded program. It can cause the initialisation of a build
child process to hang in setgroups(), as seen several times in the
build farm:

The reason is that Glibc thinks that the other threads of the parent
exist in the child, so in setxid_mark_thread() it tries to get a futex
that has been acquired by another thread just before the clone(). With
fork(), Glibc runs pthread_atfork() handlers that take care of this
(in particular, __reclaim_stacks()). But clone() doesn't do that.

Fortunately, we can use fork()+unshare() instead of clone() to set up
private namespaces.

See also https://www.mail-archive.com/lxc-devel@lists.linuxcontainers.org/msg03434.html.