pid1: fix fd memleak when we hit FileDescriptorStoreMax limit

Since service_add_fd_store() already does the check, remove the redundant check
from service_add_fd_store_set().

Also, print a warning when repopulating FDStore after daemon-reexec and we hit
the limit. This is a user visible issue, so we should not discard fds silently.
(Note that service_deserialize_item is impacted by the return value from
service_add_fd_store(), but we rely on the general error message, so the caller
does not need to be modified, and does not show up in the diff.)