run_fork_with_timeout: handle SIGTERM

when stopping a worker while doing a run_fork_with_timeout,
we want to handle that there and send the child the SIGTERM
so that it can clean up

for this we have to use readline_nointr,
because the read from the pipe gets interrupted by the signal

this partially fixes #1874
as we now correctly clean up the new disk, except if it is
on lvm/lvmthin (possibly other storages as well), and use the old disk
in the config

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>