libc: add Genode::Lock to File_descriptor class

This lock is needed to prevent multiple threads and/or multiple
libc-plugins from using the same file descriptor in p{read,write}.
libc: use File_descriptor::lock in pread/pwrite

Replace the static lock with the file descriptor lock because recursive
calls of p{read,write} would result in a deadlock when using multiple
libc-plugins at the same time.
libports: add sys/disk.h symlink
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 libfuse re-implementation

For now there is only a libc_fuse.
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 libc_fuse_ntfs-3g build file
libports: add exfat port files
libports: add libexfat build files
libports: add libc_fuse_exfat build files
libports: add fuse-ext2 port files
libports: add libext2fs build files
libports: add libc_fuse_ext2