Fix IPFilter IPv6 detection, especially on NetBSD (#596)

Fix IPFilter IPv6 detection, especially on NetBSD (#596)

Has been broken since 4eaf432.

1. acinclude/os-deps.m4 was missing sys/param.h header

2. acinclude/os-deps.m4 was incorrectly assuming that netinet/ip_nat.h
   header is always present

3. acinclude/os-deps.m4 was missing netinet/ip_nat.h header where it was
   used instead of ip_nat.h

4. src/ip/Intercept.cc assumed that AC_CHECK_MEMBERS(`struct X`)
   outputs `HAVE_X`, but it actually outputs `HAVE_STRUCT_X`.

5. acinclude/os-deps.m4: AC_CHECK_MEMBERS(`X  `) converted trailing
   whitespace into surprising trailing underscores: `HAVE_X__`