libc_fs: cleanup fd and memory in symlink()
os: extentend File_system interface with sync RPC
add sync implementation to various fs servers
noux: add SYSCALL_SYNC

File systems using the File_system_session interface can now be
synchronized by using this syscall. This is needed for file system
that maintain an internal cache, which should be flushed.
libports: prevent main symbol clash in fuse-ext2

Omit the main rountine in fuse-ext2.c. Otherwise using fuse-ext2 with
fuse_fs is not possible because the server framework already defines
a main routine.
libports: add sync method to FUSE implementation

Since file systems tend to have a inbuild caching mechansim we need to
sync these caches at the end of session when using the fuse_fs server.
Therefore each FUSE file system port has to implement a Fuse::sync_fs()
function that executes the necessary actions if requested.
libports: add symlink check to FUSE implementation
libports: less noisy fuse_ext2 compilation
libports: less noisy libext2fs compilation
libports: less noisy compilation of libfuse

Out of laziness we only provide two dummy functions for all FUSE
operations, which are used in case the FUSE file system does not
implement them itself.
libports: add libc_fuse_ntfs-3g
libports: initial fuse_fs implementation
libports: allow to check FUSE fs initialization

This changes the private API of the FUSE implementation. The init_fs
method now has to return true if the initialization was successful and
otherwise false. All current users of the API are also changed
accordingly.
ports: simple file system test running in noux