journal-remote: allow splitting incoming logs by source host

      Previously existing scheme where the file name would be based on
the source was just too ugly and unpredicatable. Now there are
only two options:
  1. just one file (until rotation),
  2. one file per source host, using the hostname as filename part.
For the cases where the source is specified by the user, only
option one is allowed, and the full of the file must be specified.