udisks2: Quote mount paths with g_shell_quote when unmounting

Currently, the unmount operation fails for mount paths that are not propagated from udisks (e.g. FUSE) and contain non-ASCII characters. This is because the mount path is escaped by the g_strescape function that drops non-ASCII characters among others. It is not clear why this function was used here, but it seems right to use the g_shell_quote function instead. Let's do so to fix unmounting in the mentioned cases.

Fixes: #706 (closed)