os: extentend File_system interface with sync RPC

Users of a File_system_session might want to force a file system
to flush or rather to synchronize its internal cache. A concret
default implementation is provided because not all file systems
maintain an internal cache and are not required to synchronize
caches.

Fixes #1007.
add sync implementation to various fs servers

Fixes #1007.
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.

Fixes #1008.
libc_fs: fix potential memory leak in symlink()

Free always the file descriptor and plugin context, return
the appropriate value to the caller and only in the error
case set errno.

Fixes #1009.