[PATCH Fix incorrect double-checked locking related to _res_hconf.initialized. [BZ #20477]

[Torvald Riegel]

_res_hconf.initialized was not suitable for use in a multi-threaded
environment due to the lack of atomics and memory barriers.  Use of it
was also unnecessary because _res_hconf_init did the right thing by
using __libc_once.  This patch fixes the glibc-internal uses by just
calling _res_hconf_init unconditionally, and switches to a release MO
atomic store for _res_hconf.initialized to fix the glibc side of the
synchronization problem (which will maintain backward compatibility, but
cannot fix the lack of acquire MO on any glibc-external loads).
    
        [BZ #20477]
        * resolv/res_hconf.c (do_init): Use atomic access.
        * resolv/res_hconf.h: Add comments.
        * nscd/aicache.c (addhstaiX): Call _res_hconf_init unconditionally.
        * nss/getXXbyYY_r.c (REENTRANT_NAME): Likewise.
        * sysdeps/posix/getaddrinfo.c (gaih_inet): Likewise.

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