NAME:
  idstring
 PURPOSE:   (one line only)
  Generate a (hopefully) unique string
 DESCRIPTION:
  This is used to provide a string that can be used to identify something
    unique to the current calculation.  The provided string can be used to
    tag things, like a unique process that is running on a given computer
    that can be used to tag and refer to different things running on the same
    (or different) machine.
 CATEGORY:
  Miscellaneous
 CALLING SEQUENCE:
  str=idstring(seed)
 INPUTS:
 OPTIONAL INPUT PARAMETERS:
  seed - optional input, seed for the random number generator.  the default
            is an undefined variable which is then passed to randomu.
 KEYWORD INPUT PARAMETERS:
  SILENT - flag, if set will suppress informative output
 OUTPUTS:
  Returns a single string with the host name, followed by '_' followed by
    4 digits to make it (nearly) unique
 KEYWORD OUTPUT PARAMETERS:
 COMMON BLOCKS:
 SIDE EFFECTS:
 RESTRICTIONS:
 PROCEDURE:
 MODIFICATION HISTORY:
  2016/12/31 - Written by Marc W. Buie, Southwest Research Institute