Replace gethostbyaddr(3) with getnameinfo(3). Remove the sigprocmask()

Replace gethostbyaddr(3) with getnameinfo(3).  Remove the sigprocmask()
that was necessary for gethostbyaddr() because the latter is not
signal safe.  Change the return code semantics of priv_getnameinfo()
to match getnameinfo(3).
input and OK jca@