global/global_init: fix stdout/stderr/stdin closing for daemonization

global/global_init: fix stdout/stderr/stdin closing for daemonization

The global_init_postfork/prefork helpers close stdout/stdin/stderr on
fork and reopen /dev/null in their place.  This ensures that if later
code writes to those descriptors (e.g., a stray cout or cerr usage) the
output/input will go nowhere instead of interfering with some other open
fd.

However, with the use of preforker, there are other threads running when
these helpers are run, which means we can race with, say, filestore
opening an object file and end up sending log output there.

Fix by atomically replacing the fds with the dup2(2) syscall, which
will implicitly close and reopen the target fd in an atomic fashion.  This
behavior is present on both Linux and FreeBSD.

Fixes: http://tracker.ceph.com/issues/23492
Signed-off-by: Sage Weil <sage@redhat.com>