resolved: properly signal transient errors back to NSS stack

resolved: properly signal transient errors back to NSS stack

NSS mostly knows four error cases: SUCCESS, NOTFOUND, UNAVAIL, TRYAGAIN,
and they can all be used in nsswitch.conf to route requests.

So far nss-resolve would return SUCCESS + NOTFOUND + UNAVAIL. Let's also
return TRYAGAIN in some cases, specifically the ones where we are
currntly unable to resolve a request but likely could later. i.e.
errors caused by networking issues or such.

Fixes: #20786
(cherry picked from commit ae78529fc75027b3487b64e0d22a572fd9c8ffeb)