netxlite: call getaddrinfo and handle platform-specific oddities (#764)

This commit changes our system resolver to call getaddrinfo directly when CGO is enabled. This change allows us to:

1. obtain the CNAME easily

2. obtain the real getaddrinfo retval

3. handle platform specific oddities such as `EAI_NODATA`
returned on Android devices

See https://github.com/ooni/probe/issues/2029 and https://github.com/ooni/probe/issues/2029#issuecomment-1140258729 in particular.

See https://github.com/ooni/probe/issues/2033 for documentation regarding the desire to see `getaddrinfo`'s retval.

See https://github.com/ooni/probe/issues/2118 for possible follow-up changes.

internal/netxlite/getaddrinfo_test.go

@@ -0,0 +1,80 @@
+package netxlite
+
+import (
+	"context"
+	"errors"
+	"io"
+	"testing"
+)
+
+func TestGetaddrinfoLookupHost(t *testing.T) {
+	addrs, err := getaddrinfoLookupHost(context.Background(), "127.0.0.1")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(addrs) != 1 || addrs[0] != "127.0.0.1" {
+		t.Fatal("unexpected addrs", addrs)
+	}
+}
+
+func TestErrorToGetaddrinfoRetval(t *testing.T) {
+	type args struct {
+		err error
+	}
+	tests := []struct {
+		name string
+		args args
+		want int64
+	}{{
+		name: "with valid getaddrinfo error",
+		args: args{
+			newErrGetaddrinfo(144, nil),
+		},
+		want: 144,
+	}, {
+		name: "with another kind of error",
+		args: args{io.EOF},
+		want: 0,
+	}}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := ErrorToGetaddrinfoRetval(tt.args.err); got != tt.want {
+				t.Errorf("ErrorToGetaddrinfoRetval() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func Test_newErrGetaddrinfo(t *testing.T) {
+	type args struct {
+		code int64
+		err  error
+	}
+	tests := []struct {
+		name string
+		args args
+	}{{
+		name: "common case",
+		args: args{
+			code: 17,
+			err:  io.EOF,
+		},
+	}}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := newErrGetaddrinfo(tt.args.code, tt.args.err)
+			if err == nil {
+				t.Fatal("expected non-nil error")
+			}
+			if !errors.Is(err, tt.args.err) {
+				t.Fatal("Unwrap() is not working correctly")
+			}
+			if err.Error() != tt.args.err.Error() {
+				t.Fatal("Error() is not working correctly")
+			}
+			if err.Code != tt.args.code {
+				t.Fatal("Code has not been copied correctly")
+			}
+		})
+	}
+}