Refactor the filesystem infrastructure

Some ideas

Please ask on the list about this topic. Some of this work has been completed.

• Optional: RTEMS might also remove ".." and "." from the path, as well as extra "/"?
• This mechanism also removes the implementation of the mount and unmount calls in each filesystem, which further reduces the complexity of writing a new filesystem. These calls are sometimes left over for years, e.g. in the MSDOS filesystem, so this is a real issue.
• The ​FUSE API is a good example.
• Support clean unmounting of filesystems (i.e. check if files are in use, and deny the unmount)
• Support forced unmounting of filesystems attached to a "dead" I/O device (e.g. an USB stick that was removed without being unmounted before)