MRD_SKIP

       Skip a number of bytes from the current location in a file or a pipe

       First tries using POINT_LUN and if this doesn't work, perhaps because
       the unit is a pipe, MRD_SKIP will just read in the requisite number
       of bytes.    GZIP files opened with /COMPRESS (in V5.3 or later) are
       also read as a series of bytes, since this is faster than using
       POINT_LUN when not at the beginning of a file.

       MRD_SKIP, Unit, Nskip

       Unit - File unit for the file or pipe in question, integer scalar
       Nskip - Number of bytes to be skipped, positive integer

       This routine should be used in place of POINT_LUN wherever a pipe
       may be the input unit (see the procedure FXPOSIT for an example).
       Note that it assumes that it can only work with nskip >= 0 so it
       doesn't even try for negative values.
       For reading a pipe, MRD_SKIP currently uses a maximum buffer size
       of 8 MB.   This chunk value can be increased for improved efficiency
       (or decreased if you really have little memory.)

       Written, Thomas A. McGlynn    July 1995
       Don't even try to skip bytes on a pipe with POINT_LUN, since this
       might reset the current pointer     W. Landsman        April 1996
       Increase buffer size, check fstat.compress W. Landsman  Jan 2001
       Only a warning if trying read past EOF   W. Landsman   Sep 2001
       Use 64bit longword for skipping in very large files W. Landsman Sep 2003

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