BUG/MEDIUM: startup: report status if daemonized process fails

BUG/MEDIUM: startup: report status if daemonized process fails

Due to master-worker rework, daemonization fork happens now before parsing
and applying the configuration. This makes impossible to report correctly all
warnings and alerts to shell's stdout. Daemonzied process fails, while being
already in background, exit code reported by shell via '$?' equals to 0, as
it's the exit code of his parent.

To fix this, let's create a pipe between parent and daemonized child. The
child will send into this pipe a "READY" message, when it finishes his
initialization. The parent will wait on the "read" end of the pipe until
receiving something. If read() fails, parent obtains the status of the
exited child with waitpid(). So, the parent can correctly report the error to
the stdout and he can exit with child's exitcode.

This fix should be backported only in 3.1.

(cherry picked from commit 663d75e7a00a9c50185cca322cbc64de934b6631)
Signed-off-by: Willy Tarreau <w@1wt.eu>