Journal: Don't use a ._ path if it won't work #5633

Journal: Don't use a ._ path if it won't work #5633

When synchronizing a folder on a samba share, creating files that begin
with ._ is often forbidden. This prevented the client from creating
its ._sync_abcdef.db file.

Now, it'll check whether the preferred filename is creatable, and if
it isn't it'll use .sync_abcdef.db instead.

The disadvantage is that this alternative path won't be ignored by
older clients - that was the reason for the ._ prefix.