Refactor dlopen() support

Nowadays, all platforms except Windows and older HP-UX have standard
dlopen() support.  So having a separate implementation per platform
under src/backend/port/dynloader/ is a bit excessive.  Instead, treat
dlopen() like other library functions that happen to be missing
sometimes and put a replacement implementation under src/port/.

Discussion: https://www.postgresql.org/message-id/flat/e11a49cb-570a-60b7-707d-7084c8de0e61%402ndquadrant.com#54e735ae37476a121abb4e33c2549b03

src/port/dlopen.c

@@ -0,0 +1,145 @@
+/*-------------------------------------------------------------------------
+ *
+ * dlopen.c
+ *	  dynamic loader for platforms without dlopen()
+ *
+ * Portions Copyright (c) 1996-2018, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *
+ * IDENTIFICATION
+ *	  src/port/dlopen.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "c.h"
+
+#if defined(__hpux)
+
+/* System includes */
+#include <a.out.h>
+#include <dl.h>
+
+void *
+dlopen(const char *file, int mode)
+{
+	int		flags = 0;
+
+	if (mode & RTLD_NOW)
+		flags |= BIND_IMMEDIATE;
+#ifdef NOT_USED
+	if (mode & RTLD_LAZY)
+		flags |= BIND_DEFERRED;
+#endif
+
+	return shl_load(filename, flags | BIND_VERBOSE, 0L);
+}
+
+void *
+dlsym(void *handle, const char *symbol)
+{
+	void	   *value;
+
+	if (shl_findsym((shl_t *) & handle, symbol, TYPE_PROCEDURE, &value) == -1)
+		return NULL;
+	return value;
+}
+
+int
+dlclose(void *handle)
+{
+	return shl_unload((shl_t) handle);
+}
+
+char *
+dlerror(void)
+{
+	static char errmsg[] = "shl_load failed";
+
+	if (errno)
+		return strerror(errno);
+
+	return errmsg;
+}
+
+#elif defined(WIN32)
+
+static char last_dyn_error[512];
+
+static void
+set_dl_error(void)
+{
+	DWORD		err = GetLastError();
+
+	if (FormatMessage(FORMAT_MESSAGE_IGNORE_INSERTS |
+					  FORMAT_MESSAGE_FROM_SYSTEM,
+					  NULL,
+					  err,
+					  MAKELANGID(LANG_ENGLISH, SUBLANG_DEFAULT),
+					  last_dyn_error,
+					  sizeof(last_dyn_error) - 1,
+					  NULL) == 0)
+	{
+		snprintf(last_dyn_error, sizeof(last_dyn_error) - 1,
+				 "unknown error %lu", err);
+	}
+}
+
+char *
+dlerror(void)
+{
+	if (last_dyn_error[0])
+		return last_dyn_error;
+	else
+		return NULL;
+}
+
+int
+dlclose(void *handle)
+{
+	if (!FreeLibrary((HMODULE) handle))
+	{
+		set_dl_error();
+		return 1;
+	}
+	last_dyn_error[0] = 0;
+	return 0;
+}
+
+void *
+dlsym(void *handle, const char *symbol)
+{
+	void	   *ptr;
+
+	ptr = GetProcAddress((HMODULE) handle, symbol);
+	if (!ptr)
+	{
+		set_dl_error();
+		return NULL;
+	}
+	last_dyn_error[0] = 0;
+	return ptr;
+}
+
+void *
+dlopen(const char *path, int mode)
+{
+	HMODULE		h;
+	int			prevmode;
+
+	/* Disable popup error messages when loading DLLs */
+	prevmode = SetErrorMode(SEM_FAILCRITICALERRORS | SEM_NOOPENFILEERRORBOX);
+	h = LoadLibrary(path);
+	SetErrorMode(prevmode);
+
+	if (!h)
+	{
+		set_dl_error();
+		return NULL;
+	}
+	last_dyn_error[0] = 0;
+	return (void *) h;
+}
+
+#endif