No way to get raw bytes for .EGU and similar fields

This is related to issue #18: after the fix applied there we get:
```
>>> caget ('LI-RF-AMPL-01:KLY:T1.EGU')
'�C'
```
as expected, but invoking `datatype=DBR_CHAR_BYTES` fails thus:
```
>>> caget ('LI-RF-AMPL-01:KLY:T1.EGU', datatype=DBR_CHAR_BYTES)
...
cothread.catools.ca_nothing: LI-RF-AMPL-01:KLY:T1.EGU: No reasonable data conversion between client and server types
```
What is going on here is that `DBR_CHAR_BYTES` retrieves the underlying PV as a `DBR_CHAR` array, and we rely on the IOC automatically converting strings (type `DBR_STRING`) to character arrays. Unfortunately it seems that the IOC doesn't do this for `.EGU`, and possibly other fields.