gvfs-mount: allow specifying G_MOUNT_UNMOUNT_FORCE

https://bugzilla.gnome.org/show_bug.cgi?id=710986
gvfsjobunmount: set error as per client interaction

If the dialog about blocking processes was cancelled,
G_IO_ERROR_FAILED_HANDLED will be returned. If there were outstanding
jobs and GMountOperation wasn't passed in, or "show-processes" signal
wasn't handled, G_IO_ERROR_BUSY will be returned.

https://bugzilla.gnome.org/show_bug.cgi?id=710986