tailcfg: de-pointer a bunch of fields now that we have omitzero

tailcfg.go is full of pointers to things just for JSON `omitempty` reasons. Nowadays we would've dropped the pointer and used `omitzero` instead.

We should do a pass over tailcfg and the caller code and see if we can modernize them.

We'll have to be careful on the control server side to not break old clients (or the server handling requests from old clients), but maybe that's easier than I'm fearing.

/cc @dsnet @tailscale/client-core