Merge ifgroup and interface name namespaces

[kp]

Summary

pf allows network interfaces and groups to be used interchangeably. They don't
share a namespace so it's possible for an interface and a group to have the same
name.
Merge the namespaces, because this fixes both the panic in pf and brings us closer to admin expectations.

PR: 127042, 202178

[kp]

I'm not very happy with this patch, both for locking reasons and error handling reasons. I'm not sure how to improve it however, and wanted to post it in case people had better ideas.