core: add a new sd_notify() message for removing fds from the FD store again

core: add a new sd_notify() message for removing fds from the FD store again

Currenly the only way to remove fds from the fdstore is to fully
stop the service, or to somehow trigger POLLERR/POLLHUP on the fd, in
which case systemd will remove the fd automatically.

Let's add another way: a new message that can be sent to remove fds
explicitly, given their name.