ieee488: introduce 'delim' option for improved text output

When a capture contains output from a talking device, but neither
contains EOI (end of transmission) nor ATN (new commands, including
changes in the set of communicating peers or communication direction),
then lower layer data bytes would be shown but accumulated runs of upper
layer payload data would not. The hp53131a-ton.sr capture demonstrates
this behaviour.

Add transparent support for the typical case of communicating SCPI over
GPIB. Do emit upper layer payload data annotations when termination
sequences for text lines were seen. Allow users to disable this feature
when it's not appropriate for binary content.

It's an internal implementation detail that accumulated data gets flushed.
Communicate the user visible option as "payload data separation", which
should most appropriately reflect the resulting behaviour. The specific
implementation of this feature can also support different kinds of payload
chunk separation.