Don't leak a signalfd when using latches in the postmaster.

Don't leak a signalfd when using latches in the postmaster.

At the time of commit 6a2a70a02 we didn't use latch infrastructure in
the postmaster.  We're planning to start doing that, so we'd better make
sure that the signalfd inherited from a postmaster is not duplicated and
then leaked in the child.

Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Justin Pryzby <pryzby@telsasoft.com>
Discussion: https://postgr.es/m/CA%2BhUKG%2BZ-HpOj1JsO9eWUP%2Bar7npSVinsC_npxSy%2BjdOMsx%3DGg%40mail.gmail.com