lib: rearm pre-existing signal event registrations

process.on('somesignal', ...) semantics expect the process to catch the
signal and invoke the associated handler. `setupSignalHandlers` perform
the additional task of preparing the libuv signal handler and associate
it with the event handler. It is possible that by the time this is
setup there could be pre-existing registrations that pre-date this setup
in the boot sequence.

So rearm pre-existing signal event registrations to get upto speed.

Ref: https://github.com/nodejs/node/pull/22712#discussion_r232457318

PR-URL: https://github.com/nodejs/node/pull/24651


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>