* [#16499](https://github.com/apache/nuttx/pull/16499) [BREAKING] fs/vfs: Separate file descriptors from file descriptions

This PR is a rework of the NuttX file descriptor implementation. The goal is two-fold:

Improve POSIX compliance. The old implementation tied file description
to inode only, not the file struct. POSIX however dictates otherwise.
Fix a bug with descriptor duplication (dup2() and dup3()). There is
an existing race condition with this POSIX API that currently results
in a kernel side crash.
The crash occurs when a partially open / closed file descriptor is
duplicated. The reason for the crash is that even if the descriptor is
closed, the file might still be in use by the kernel (due to e.g. ongoing
write to file). The open file data is changed by file_dup3() and this
causes a crash in the device / drivers themselves as they lose access to
the inode and private data.

The fix is done by separating struct file into file and file descriptor
structs. The file struct can live on even if the descriptor is closed,
fixing the crash. This also fixes the POSIX issue, as two descriptors
can now point to the same file.

The implementation of this PR is based on the modifications made in #16361.
Thank you @pussuw

DEPENDS ON: apache/nuttx-apps#3091

IMPACT

Remove the FS_REFCOUNT config because reference counting is a very necessary feature 
that is required in many scenarios to ensure stability.
Rename the functions fs_getfilep, fs_putfilep, and fs_reffilep to file_get, file_put, 
and file_ref respectively, to unify the naming convention to file_xxx, such as file_open,
 file_ioctl, etc. 
Introduce a new fd (file descriptor) structure. If multiple file descriptors (fds) are in
 a dup relationship, they can share the same file entity to implement the dup functionality.
Modify the functions nx_close_from_tcb, nx_open_from_tcb, nx_dup2_from_tcb, 
and nx_dup3_from_tcb to fdlist_open, fdlist_close, fdlist_dup2, and fdlist_dup3 respectively,
 to uniformly operate on file descriptors using fdlist.