Set EAI_SYSTEM only when h_errno is NETDB_INTERNAL

Fixes BZ #15339.

NSS_STATUS_UNAVAIL may mean that a necessary input resource is not
available.  This could occur in a number of cases including when the
network is down, system runs out of file descriptors, etc.  The
correct differentiator in such a case is the h_errno, which gives the
nature of failure.  In case of failures other than a simple 'not
found', we set h_errno as NETDB_INTERNAL and let errno be the
identifier for the exact error.