getnetconfig: make nc_error a thread local variable

[brooks]

Summary

Remove a bunch of complexity at the cost of 4 bytes of storage per
thread.

Sponsored by: DARPA, AFRL

[jhb]

I suspect the complexity wasn't to avoid 4 bytes per thread but more because TLS wasn't reliably working on all of our architectures when this was first written (but I could be wrong). :)

[brooks]

In D55310#1269811, @jhb wrote:

I suspect the complexity wasn't to avoid 4 bytes per thread but more because TLS wasn't reliably working on all of our architectures when this was first written (but I could be wrong). :)

I'm sure that's true, it just turns out to be a small benefit in the common case. There are a bunch more of these that I'm tempted to convert, but they involve large structs.