Fix portability issues in new src/port/inet_net_ntop.c file.

Fix portability issues in new src/port/inet_net_ntop.c file.

1. Don't #include postgres.h in a frontend build.

2. Don't assume that the backend's symbol PGSQL_AF_INET6 has anything to do
with the constant that will be used by system library functions (because,
in point of fact, it usually doesn't).  Fortunately, PGSQL_AF_INET is equal
to AF_INET, so we can just cater for both sets of values in one case
construct without fear of conflict.