This moves the write_offset variable from struct netconn to struct api_msg
This optimizes the storage by only having the space claimed when it is
needed (during a netconn_write_partly() call) and not throughout the
lifetime of the netconn
This also reduces code space/execution by not having to separately manage
clearing/checking write_offset from the current_msg pointer
Lastly, we also save execution by using msg.w.offset as the output
rather than marshaling the result to msg.w.len. Previously, len was used
as input length of dataptr and output for the write operation.
netconn_write_partly() also has access to msg.w.offset, so we can use
that
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