Avoid use of Perl getprotobyname

Avoid use of Perl getprotobyname

getprotobyname returns undefined on some CI machines.  It's not clear
why.  The code overall still works, but it raises a warning.

In PostgreSQL C code, we always call socket() with 0 for the protocol
argument, so we should be able to do the same in Perl (since the Perl
documentation says that the arguments of the socket function are the
same as in C).  So do that, to avoid the issue.

Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://www.postgresql.org/message-id/flat/06f899fd-1826-05ab-42d6-adeb1fd5e200%40eisentraut.org