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/DirectoryReader.hxx

@@ -0,0 +1,87 @@
+/*
+ * 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.
+ */
+
+#ifndef MPD_FS_DIRECTORY_READER_HXX
+#define MPD_FS_DIRECTORY_READER_HXX
+
+#include "check.h"
+#include "Path.hxx"
+
+#include <dirent.h>
+
+/**
+ * Reader for directory entries.
+ */
+class DirectoryReader {
+	DIR *const dirp;
+	dirent *ent;
+public:
+	/**
+	 * Creates new directory reader for the specified #dir.
+	 */
+	explicit DirectoryReader(const Path &dir)
+		: dirp(opendir(dir.c_str())),
+		  ent(nullptr) {
+	}
+
+	DirectoryReader(const DirectoryReader &other) = delete;
+	DirectoryReader &operator=(const DirectoryReader &other) = delete;
+
+	/**
+	 * Destroys this instance.
+	 */
+	~DirectoryReader() {
+		if (!Failed())
+			closedir(dirp);
+	}
+
+	/**
+	 * Checks if directory failed to open. 
+	 */
+	bool Failed() const {
+		return dirp == nullptr;
+	}
+
+	/**
+	 * Checks if directory entry is available.
+	 */
+	bool HasEntry() const {
+		assert(!Failed());
+		return ent != nullptr;
+	}
+
+	/**
+	 * Reads next directory entry.
+	 */
+	bool ReadEntry() {
+		assert(!Failed());
+		ent = readdir(dirp);
+		return HasEntry();
+	}
+
+	/**
+	 * Extracts directory entry that was previously read by #ReadEntry.
+	 */
+	Path GetEntry() const {
+		assert(HasEntry());
+		return Path::FromFS(ent->d_name);
+	}
+};
+
+#endif