unix: Support MICROPY_VFS_POSIX and enable it in coverage build.

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.