BUG/MINOR: mworker: reintroduce way to disable seamless reload with -x /dev/null

BUG/MINOR: mworker: reintroduce way to disable seamless reload with -x /dev/null

Since the introduction of the automatic seamless reload using the
internal socketpair, there is no way of disabling the seamless reload.

Previously we just needed to remove -x from the startup command line,
and remove any "expose-fd" keyword on stats socket lines.

This was introduced in 2be557f7c ("MEDIUM: mworker: seamless reload use
the internal sockpairs").

The patch copy /dev/null again and pass it to the next exec so we never
try to get socket from the -x.

Must be backported as far as 2.6.