MINOR: channel: Add functions to get info on buffers and deal with HTX streams

MINOR: channel: Add functions to get info on buffers and deal with HTX streams

This patch adds HXT-aware versions of the functions c_data(), ci_data() and
c_empty(). channel_data() function returns the amount of data in the
channel, channel_input_data() returns the amount of input data and
channel_empty() returns true if the channel's buffer is empty. These
functions handles HTX buffers.

In addition, channel_data_limit() function, still HTX-aware, can be used to
get the maximum absolute amount of data that can be copied in a buffer,
independently on data already present in the buffer.