nscd: Do not use __inet_aton_exact@GLIBC_PRIVATE [BZ #20018]

This commit avoids referencing the __inet_aton_exact@GLIBC_PRIVATE
symbol from nscd.  In master, the separately-compiled getaddrinfo
implementation in nscd needs it, however such an internal ABI change
is not desirable on a release branch if it can be avoided.