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

Makefile.am

@@ -151,6 +151,8 @@ src_mpd_SOURCES = \
 	src/thread/PosixCond.hxx \
 	src/thread/WindowsCond.hxx \
 	src/thread/GLibCond.hxx \
+	src/fs/FileSystem.cxx src/fs/FileSystem.hxx \
+	src/fs/DirectoryReader.hxx \
 	src/glib_socket.h \
 	src/clock.c src/clock.h \
 	src/notify.cxx src/notify.hxx \
@@ -289,7 +291,6 @@ src_mpd_SOURCES = \
 	src/Volume.cxx src/Volume.hxx \
 	src/SongFilter.cxx src/SongFilter.hxx \
 	src/PlaylistFile.cxx src/PlaylistFile.hxx \
-	src/FileSystem.cxx src/FileSystem.hxx \
 	src/timer.c
 
 #