net: rework the CNAME handling on unix

[mateusz834]

This change reworks the LookupCNAME handling:

1. It removes the fallback to the go resolver
   when cgoLookupCNAME faills (even noSuchHosts), this
   also implies using the canonical name from getaddrinfo.
2. Improves the error handling of the res_search cgo routines, so
   returning proper &DNSError to match the go resolver behaviour and
   detecting the noSuchHost condition.
3. Makes the LookupCNAME return the last CNAME in the chain,
   not the first one.

More details about current issues are described in #59943.

Fixes #59943
Updates #50101

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