iptables and quagga need them to work.
according to the documentation in the man pages, the GNU extension functions gethostbyaddr_r, gethostbyname_r and gethostbyname2_r are guaranteed to set the result pointer to NULL in case of error or no result.
this case is not even documented, but the kernel returns 0 here and it makes sense to be consistent.
these are not pure syscall wrappers because they have to work around kernel API bugs on 64-bit archs. the workarounds could probably be made somewhat more efficient, but at the cost of more complexity. this may be revisited later.
the results of a dns query, whether it's performed as part of one of the standard name-resolving functions or directly by res_send, should be a function of the query, not of the particular nameserver that responds to it. thus, all responses which indicate a failure or refusal by the nameserver, as opposed to a positive or negative result for the query, should be ignored. the strategy used is to re-issue the query immediately (but with a limit on the number of retries, in case the server is really broken) when a response code of 2 (server failure, typically transient) is seen, and otherwise take no action on bad responses (which generally indicate a misconfigured nameserver or one which the client does not have permission to use), allowing the normal retry interval to apply and of course accepting responses from other nameservers queried in parallel. empirically this matches the traditional resolver behavior for nameservers that respond with a code of 2 in the case where there is just a single nameserver configured. the behavior diverges when multiple nameservers are available, since musl is querying them in parallel. in this case we are mildly more aggressive at retrying.
the fcntl function is heavy, so make the syscall directly instead. also, avoid the code size and runtime overhead of querying the old flags, since it's reasonable to assume nothing will be set on a newly-created socket. this code is only used on old kernels which lack proper atomic close-on-exec support, so future changes that might invalidate such an assumption do not need to be considered.
as usual, this is non-atomic, but better than producing an error or failing to set the close-on-exec flag at all.
the input name is validated, the other parameters are assumed to be valid (the list of already compressed names are not checked for infinite reference loops or out-of-bound offsets). names are handled case-sensitively for now.
trailing . should be accepted in domain name strings by convention (RFC 1034), host name lookup accepts "." but rejects empty "", res_* interfaces also accept empty name following existing practice.