lxc_attach: Use clone() instead of second fork()

Because of an assertion in glibc's fork() wrapper that parent pid and
pid of child should never be the same, one should avoid fork() after
attaching to a PID namespace, since the pid inside the namespace may
coincide with the pid of the parent outside the namespace, thus hitting
the aforementioned assertion.

This patch just changes the code in the most simple manner to use
clone() instead of fork(). Since clone() requires a function to be
called instead of returning 0, we move the code of the child into a
function child_main.

Signed-off-by: Christian Seiler <christian@iwakd.de>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>