Phil Pennock*sigh* FreeNAS appear to be building packages on newer base image without changing the pkg ABI definition, so `pkg upgrade` broke zsh because `libdl.so .1` is missing. 5.6.2_1 bad, 5.6.2 good. `pkg install /var/cache/pkg/zsh-5.6.2.txz` recovers.
Oh. One of the jails is stuck on an old userland and FreeNAS's messing around with jail ecosystems has left this one behind. Time to figure out how to kick it back into manageability.