Move a couple of initdb's subroutines into src/port/.

mkdir_p and check_data_dir will be useful in CREATE TABLESPACE, since we
have agreed that that command should handle subdirectory creation just like
initdb creates the PGDATA directory.  Push them into src/port/ so that they
are available to both initdb and the backend.  Rename to pg_mkdir_p and
pg_check_dir, just to be on the safe side.  Add FreeBSD's copyright notice
to pgmkdirp.c, since that's where the code came from originally (this
really should have been in initdb.c).  Very marginal code/comment cleanup.

src/port/pgcheckdir.c

@@ -0,0 +1,73 @@
+/*-------------------------------------------------------------------------
+ *
+ * src/port/pgcheckdir.c
+ *
+ * A simple subroutine to check whether a directory exists and is empty or not.
+ * Useful in both initdb and the backend.
+ *
+ * Portions Copyright (c) 1996-2010, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "c.h"
+
+#include <dirent.h>
+
+
+/*
+ * Test to see if a directory exists and is empty or not.
+ *
+ * Returns:
+ *		0 if nonexistent
+ *		1 if exists and empty
+ *		2 if exists and not empty
+ *		-1 if trouble accessing directory (errno reflects the error)
+ */
+int
+pg_check_dir(const char *dir)
+{
+	int			result = 1;
+	DIR		   *chkdir;
+	struct dirent *file;
+
+	errno = 0;
+
+	chkdir = opendir(dir);
+
+	if (chkdir == NULL)
+		return (errno == ENOENT) ? 0 : -1;
+
+	while ((file = readdir(chkdir)) != NULL)
+	{
+		if (strcmp(".", file->d_name) == 0 ||
+			strcmp("..", file->d_name) == 0)
+		{
+			/* skip this and parent directory */
+			continue;
+		}
+		else
+		{
+			result = 2;			/* not empty */
+			break;
+		}
+	}
+
+#ifdef WIN32
+
+	/*
+	 * This fix is in mingw cvs (runtime/mingwex/dirent.c rev 1.4), but not in
+	 * released version
+	 */
+	if (GetLastError() == ERROR_NO_MORE_FILES)
+		errno = 0;
+#endif
+
+	closedir(chkdir);
+
+	if (errno != 0)
+		result = -1;			/* some kind of I/O error? */
+
+	return result;
+}