iphlpapi: _res is per-thread in glibc with pthreads (try 2)

[Dan Kegel]

With test.  Without the fix, the test case crashes on my machine
if you add more than one nameserver in /etc/resolv.conf.
Fixes crash in bug http://bugs.winehq.org/show_bug.cgi?id=21624

Same fix probably needed in dnsapi, or maybe iphlpapi should
use dnsapi rather than unix dns api.

Thanks to AF for the analysis.

Change since first posted: much simpler, use the init
bit built into the resolver.
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