Use localhost for default unix domain socket authority

Only tested with my local build against a Rust tonic-based gRPC server:

% ~/go/bin/grpcurl -vv -d @ -plaintext -unix -protoset ~/REDACTED/_descriptor.bin ~/REDACTED/api.socket REDACTED/Reverse <<EOM
heredoc> { "text": "Hello, world!" }
heredoc> EOM

Resolved method descriptor:
rpc Reverse ( .REDACTED.ReverseRequest ) returns ( .REDACTED.ReverseResponse );

Request metadata to send:
(empty)

Response headers received:
content-type: application/grpc
date: Tue, 30 Jan 2024 16:00:24 GMT

Estimated response size: 15 bytes

Response contents:
{
  "text": "!dlrow ,olleH"
}

Response trailers received:
(empty)
Sent 1 request and received 1 response
Timing Data: 16.914667ms
  Dial: 9.9495ms
    BlockingDial: 9.92875ms
  InvokeRPC: 5.544584ms

fullstorydev / grpcurl

Use localhost for default unix domain socket authority #445