Now that the coverage build has fully switched to the VFS sub-system these
functions were no longer available, so add them to the uos_vfs module.
Also, vfs_open is no longer needed, it's available as the built-in open.
The unix coverage build is now switched fully to the VFS implementation, ie
the uos module is the uos_vfs module. For example, one can now sandbox uPy
to their home directory via:
$ ./micropython_coverage
>>> import uos
>>> uos.umount('/') # unmount existing root VFS
>>> vfs = uos.VfsPosix('/home/user') # create new POSIX VFS
>>> uos.mount(vfs, '/') # mount new POSIX VFS at root
Some filesystem/OS features may no longer work with the coverage build due
to this change, and these need to be gradually fixed.
The standard unix port remains unchanged, it still uses the traditional uos
module which directly accesses the underlying host filesystem.
This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
