re-integrate nextstep dynloader functionality

From: Jacek Lasecki <jacek@sound.eti.pg.gda.pl>

src/backend/port/dynloader/nextstep.c

@@ -0,0 +1,81 @@
+#include <mach-o/rld.h>
+#include <streams/streams.h>
+#include <stdlib.h>
+
+static char *lastError = NULL;
+
+static NXStream *
+OpenError()
+{
+	return NXOpenMemory(NULL, 0, NX_WRITEONLY);
+}
+
+static void
+CloseError(NXStream * s)
+{
+	if (s)
+		NXCloseMemory(s, NX_FREEBUFFER);
+}
+
+static void
+TransferError(NXStream * s)
+{
+	char	   *buffer;
+	int			len,
+				maxlen;
+
+	if (lastError)
+		free(lastError);
+	NXGetMemoryBuffer(s, &buffer, &len, &maxlen);
+	lastError = malloc(len + 1);
+	strcpy(lastError, buffer);
+}
+
+void *
+next_dlopen(char *name)
+{
+	int			rld_success;
+	NXStream   *errorStream;
+	char	   *result = NULL;
+	char	  **p;
+
+	errorStream = OpenError();
+	p = calloc(2, sizeof(void *));
+	p[0] = name;
+	rld_success = rld_load(errorStream, NULL, p, NULL);
+	free(p);
+
+	if (!rld_success)
+	{
+		TransferError(errorStream);
+		result = (char *) 1;
+	}
+	CloseError(errorStream);
+	return result;
+}
+
+int
+next_dlclose(void *handle)
+{
+	return 0;
+}
+
+void *
+next_dlsym(void *handle, char *symbol)
+{
+	NXStream   *errorStream = OpenError();
+	char		symbuf[1024];
+	unsigned long symref = 0;
+
+	sprintf(symbuf, "_%s", symbol);
+	if (!rld_lookup(errorStream, symbuf, &symref))
+		TransferError(errorStream);
+	CloseError(errorStream);
+	return (void *) symref;
+}
+
+char *
+next_dlerror(void)
+{
+	return lastError;
+}