[PATCH] Fix double-checked locking in _res_hconf_reorder_addrs [BZ #19074]

[Florian Weimer]

This addresses a data race in libresolv.  The race is not entirely
benign, it could cause programs to access a partially initialized
ifaddrs array.

I made sure this compiles on x86_64, but this code is difficult to test.

A potential follow-up optimization would be to move the socket creation
under the lock.  No need to create a socket if we lose the race to the lock.

Florian
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