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

[Araneidae]

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.

[Araneidae]

This can probably be fixed by changing the behaviour of DBR_CHAR_BYTES and its allies, by first checking the underlying record datatype and treating DBR_STRING separately.