line extractor: Assert a not null charset.

Start a work around a dctc dispatch bug.

* src/core/java/com/dataiku/dip/input/formats/LineFormatExtractor.java:
  Here.
dispatch: Add a charset option.

* src/dctc/java/com/dataiku/dctc/command/Dispatch.java: Could be
  needed.
dispatch: Initialize the file number to -1.

* src/dctc/java/com/dataiku/dctc/command/Dispatch.java: Forgotten
  init.