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

By running one of these run scripts a user-defined shell script will
be executed within in noux, which conducts the defined file systems
tests for given File_system_session based file system.
noux: add mkfs test scenario
os: add read-only option part_blk XXX

XXX this change only prevents part_blk from setting the operation of
Packet_descriptor::WRITE.