libports: libc_block plugin

Use a Block_session directly through the libc. Currently only the
functions close, lseek, open, read and write are implemented.
libports: add ntfs-3g port files
libports: add libntfs-3g build files

Direct Block_session access is implemented in genode_io.cc but is not
used at the moment.
libports: add libfuse re-implementation

For now there is only a libc_fuse.
libports: add libc_fuse_ntfs-3g build file
libports: add exfat port files
libports: add libexfat build files
libports: add libc_fuse_exfat build files