* lib/arpa_inet.in.h: Include c++defs.h.
(inet_ntop, inet_pton): Declare using the macros with C++ namespace
support.
* modules/arpa_inet (Depends-on): Add c++defs.
(Makefile.am): Substitute the contents of c++defs.h.
* modules/arpa_inet-tests (Depends-on): Add arpa_inet-c++-tests.
* modules/arpa_inet-c++-tests: New file.
* tests/test-arpa_inet-c++.cc: New file.
