FileSystem: extract DirectoryReader, improve the rest

- DirectoryReader is extracted to separate header
- FileSystem.cxx/FileSystem.hxx/DirectoryReader.hxx moved to fs/ subdir
- Functions return true on success, instead of 0 (where applicable)
- ReadLink return result instead of out parameter
- UnlinkFile is renamed to RemoveFile
- CheckExists/CheckIsRegular/CheckIsDirectory are renamed
  to PathExists/FileExists/DirectoryExists

src/fs/FileSystem.cxx

@@ -0,0 +1,43 @@
+/*
+ * Copyright (C) 2003-2013 The Music Player Daemon Project
+ * http://www.musicpd.org
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+ */
+
+#include "config.h"
+#include "FileSystem.hxx"
+
+#include <errno.h>
+
+Path ReadLink(const Path &path)
+{
+#ifdef WIN32
+	(void)path;
+	errno = EINVAL;
+	return Path::Null();
+#else
+	char buffer[MPD_PATH_MAX];
+	ssize_t size = readlink(path.c_str(), buffer, MPD_PATH_MAX);
+	if (size < 0)
+		return Path::Null();
+	if (size >= MPD_PATH_MAX) {
+		errno = ENOMEM;
+		return Path::Null();
+	}
+	buffer[size] = '\0';
+	return Path::FromFS(buffer);
+#endif
+}