tappet.in: Simplify and improve exporting

* tappet.in (export-log!, export-month!, export-year!)
  (export-channel!, main): Simplify and improve exporting.
  (export-channels!): New procedure.
  (write-index!): Change arguments.  All callers updated.
  (write-index/files!): Remove.
tappet.in (make-html-footer): Add a timestamp

* tappet.in (make-html-footer): Add a timestamp
  (timestamp->link): Update.
log.scm (log-line): Use `list->dsv-string'

* log.scm (log-line): Use `list->dsv-string' to export lines so colons
  in messages will be escaped.
  (%log-file-format-version): Bump version to 2