run_fork_with_timeout: do not overwrite global signal handlers

perls 'local' must be either used in front of each $SIG{...}
assignments or they must be put in a list, else it affects only the
first variable and the rest are *not* in local context.

This may cause weird behaviour where daemons seemingly do not get
terminating signals delivered correctly and thus may not shutdown
gracefully anymore.

As we only send SIGINT to processes if a manual stop action gets
triggered just catch this one here.

As this is a general method which allows to pass an arbitrary code
payload we cannot sanely handle all signals here, so remove trapping
all other besides SIGINT, if those need to be trapped that should be
done by the caller on a case by case basis.

Fixes: #1495
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>