Port inet_ntop to Solaris 8 (problem found in coreutils 7.2).

Port inet_ntop to Solaris 8 (problem found in coreutils 7.2).

* modules/arpa_inet (arpa/inet.h): Depend on arpa_inet.in.h.
* m4/inet_ntop.m4 (gl_INET_NTOP): Search for inet_ntop in -lnsl,
for Solaris 8.  This is a bit of a hack, as it means it's the
caller's responsibility to add -lnsl if needed, but most likely it
won't be needed since only getaddrinfo uses this and getaddrinfo
isn't needed on Solaris 8.