nspawn: check return of setsid()

nspawn: check return of setsid()

Let's verify that everything works the way we expect it to work, hence
check setsid() return code.

(cherry picked from commit efe426624046089b74b70708e61f304f3cd414a7)
(cherry picked from commit 6f9c8af500fd4362d656917703eab4f941e7e45a)
(cherry picked from commit a049ddfc61e20b19fbb64d59602e25e8ccad140e)