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.go

@@ -0,0 +1,56 @@
+package netxlite
+
+import (
+	"context"
+	"errors"
+)
+
+// getaddrinfoLookupHost performs a DNS lookup and returns the
+// results. If we were compiled with CGO_ENABLED=0, then this
+// function calls net.DefaultResolver.LookupHost. Otherwise,
+// we call getaddrinfo. In such a case, if getaddrinfo returns a nonzero
+// return value, we'll return as error an instance of the
+// ErrGetaddrinfo error. This error will contain the specific
+// code returned by getaddrinfo in its .Code field.
+func getaddrinfoLookupHost(ctx context.Context, domain string) ([]string, error) {
+	addrs, _, err := getaddrinfoLookupANY(ctx, domain)
+	return addrs, err
+}
+
+// ErrGetaddrinfo represents a getaddrinfo failure.
+type ErrGetaddrinfo struct {
+	// Err is the error proper.
+	Underlying error
+
+	// Code is getaddrinfo's return code.
+	Code int64
+}
+
+// newErrGetaddrinfo creates a new instance of the ErrGetaddrinfo type.
+func newErrGetaddrinfo(code int64, err error) *ErrGetaddrinfo {
+	return &ErrGetaddrinfo{
+		Underlying: err,
+		Code:       code,
+	}
+}
+
+// Error returns the underlying error's string.
+func (err *ErrGetaddrinfo) Error() string {
+	return err.Underlying.Error()
+}
+
+// Unwrap allows to get the underlying error value.
+func (err *ErrGetaddrinfo) Unwrap() error {
+	return err.Underlying
+}
+
+// ErrorToGetaddrinfoRetval converts an arbitrary error to
+// the return value of getaddrinfo. If err is nil or is not
+// an instance of ErrGetaddrinfo, we just return zero.
+func ErrorToGetaddrinfoRetval(err error) int64 {
+	var aierr *ErrGetaddrinfo
+	if err != nil && errors.As(err, &aierr) {
+		return aierr.Code
+	}
+	return 0
+}