CANopen bugfixes

1. Fixed endianess in CANopen dissector.
According to CiA 301, 7.1.1. (p. 26):
"For numerical data types the encoding is little endian style."

2. Fixed NMT type string in CANopen dissector
NMT function code should not display 'EMERGENCY'

3. Fixed time stamp decoding

* Offset increment was too low for data type size
* Decoding of time_stamp_days must equal time_stamp_msec and thus be letohs instead of ntohs. CANopen data is little-endian encoded.

4. Fix: Use correct description string for NMT error control state bits
canopen.nmt_guard.state was faulty named "Node-ID". This was changed to "State".

5. Fix nmt_guard_state value_string array
CiA 301 desribes only 4 valid values. All other were deleted.
0x00 was renamed from 'Initalisation' to 'Boot-up' following CiA301.

6. Shortened EMERGENCY to EMCY

The term EMCY is the standard abbreviation used in CiA standard for Emergency service.

7. Fix: Allow SYNC and NMT error frames without any payload
NMT node guard remote requests do note have a payload,
SYNC frames only have an optional payload (counter)

If item length is set to -1, decode will cause a 'Malformed Packet' error.

8.  Rename MT_NMT_GUARD to MT_NMT_ERR_CTRL which better reflects its scope

Change-Id: I676f9b5f2e4efd8e7c9528fe289e7510c4d43235
Reviewed-on: https://code.wireshark.org/review/5425
Reviewed-by: Michael Mann <mmann78@netscape.net>
Petri-Dish: Michael Mann <mmann78@netscape.net>
Reviewed-by: Evan Huus <eapache@gmail.com>