modbus: Make C->S and S->C configurable, add framegap option.

modbus: Make C->S and S->C configurable, add framegap option.

Change client->server and server->client to be separately configurable,
allowing decoding at both the server (where client->server is RX and
server->client is TX) and client (where client->server is TX and
server->client is RX) ends of the link. It also allows monitoring of the
bus on a single channel (where client->server and server->client are both
RX (or TX)).

When I tried to decode a bus capture, I found that when the transmitter was
turned off it generated a false start bit, which in turn resulted in a false
trailing byte from the UART decoder. This narrowed the inter-frame gap to
the point where the Modbus decoder failed to recognise a new frame. The
result was only the first frame of the capture decoded - all the rest of the
frames failed to decode. I had to reduce the frame gap to allow subsequent
frames to decode, and so made it a configurable option that defaults to the
existing gap.

Lastly, I fixed a call to puti() that incorrectly included the annotation
prefix.