resolve: enforce the search domain limit earlier (#41938)

The search domain limit is already enforced by dns_search_domain_new(),
but in this case it's way too late. Let's enforce it during the first
loop to avoid unnecessary parsing.

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Also, set a similar limit for NTAs - introduce a new constant, since
there's no pre-existing limit. I pulled the value from a thin air since
there's (AFAIK) no mandated maximum/minimum for NTAs, but given they're
supposed to be a manual and _temporary_ workarounds, hopefully 2K of
NTAs will be more than enough (if not, please yell).

Also note: the newly added error messages don't have the trailing "."
and similarly the newly introduced constant doesn't have the "u" suffix
to match the style of the surrounding code (and I didn't want to fix the
surrounding code to make the diff minimal). If this is not desirable,
please also yell.