Add routines for CRC-16 with a polynomial of 0x3D65.

There are routines that take a buffer and a length and that take a
tvbuff, offset, and length; use those routines in the DNP dissector
(which no longer needs its own table and loop), and use the tvbuff
routine instead of calling tvb_get_ptr().

Change-Id: Ic67b0f3b65b94ea47c0fdc2f3d3b6f88df77f9c6
Reviewed-on: https://code.wireshark.org/review/3505
Reviewed-by: Guy Harris <guy@alum.mit.edu>