Adding a pg_servername() function

I used shamelessly the idea and created a working proof of concept:

- the function takes no argument and returns the hostname or a null value if any error occurs

- the function was added to the network.c file because it makes sense to me to have that near the inet_server_addr() and inet_server_port() functions.

- I didn't add any test as the inet functions are not tested either.

If you think my design is good enough, I'll go ahead and port/test that function for Windows.

Have a nice day,

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