MINOR: buffer: add two functions to inject data into buffers

MINOR: buffer: add two functions to inject data into buffers

bi_istput() injects the ist string into the input region of the buffer,
it will be used to feed small data chunks into the conn_stream. bo_istput()
does the same into the output region of the buffer, it will be used to send
data via the transport layer and assumes there's no input data.