Add fields of all SD-ELEMENTS, not just the first.

Before this only the fields of the first RFC5424 SD-ELEMENT have been
added to the fields map. This is fixed now by iterating over all
existing SD-ELEMENTS returned by the syslog parser.

Known problem:

If two different SD-ELEMENTs share the same key in the SD-PARAMS, the
last value wins. See code comments for details.