Use the datalog device to inspect data segments.  Connect it inline in
a dataflow (both the source and sink are connected) to log data
segment information as the data passes through it, or connect it at
the end of a dataflow (just the sink connected).

The device has four modes, settable using mas_set "mode" with a
packaged string argument.  "mode" can be one of:

"quiet" - nothing is logged.
"header" - the data segment headers are logged.
"hex" - hexdumps of the data segments are logged
"header hex" - headers and hexdumps of the data segments are logged
"raw" - the raw data segment contents are logged.  See below.

Additionally, you may set a filename to which output will be logged,
instead of the MAS log.  This is a requirement for the "raw" mode,
which will not log raw packet data to the MAS log.  To use it, mas_set
"filename" with a packaged string argument "filename".  Note that the
filenames may not be cross-platform compatible.

