This is a fix left over from the v2 conversion. Dropbox ignores the
client modification on an incoming file if it was identical to the
existing file. This change deletes the existing file first before
re-uploading the new one.
Normally mount/cmount use `-o ro` to get the kernel to mark the fs as
read only. However this is ignored by WinFsp, so in addition if
`--read-only` is in effect then return EROFS ("Read only File System")
from all methods which attempt to modify something.
