Fix potential hanging of gethostbyaddr_r/gethostbyname_r

Fix potential hanging of gethostbyaddr_r/gethostbyname_r

When "reorder" resolver option is enabled, threads of a multi-threaded process
could hang in gethostbyaddr_r, gethostbyname_r, or gethostbyname2_r.

Due to a trivial bug in _res_hconf_reorder_addrs, simultaneous
invocations of this function in a multi-threaded process could result to
_res_hconf_reorder_addrs returning without releasing the lock it holds,
causing other threads to block indefinitely while waiting for the lock
that is not going to be released.

[BZ #17977]
* resolv/res_hconf.c (_res_hconf_reorder_addrs): Fix unlocking
when initializing interface list, based on the bug analysis
and the patch proposed by Eric Newton.
* resolv/tst-res_hconf_reorder.c: New test.
* resolv/Makefile [$(have-thread-library) = yes] (tests): Add
tst-res_hconf_reorder.
($(objpfx)tst-res_hconf_reorder): Depend on $(libdl)
and $(shared-thread-library).
(tst-res_hconf_reorder-ENV): New variable.