[MINOR] buffers: provide more functions to handle buffer data

[MINOR] buffers: provide more functions to handle buffer data

buffer_contig_space(), buffer_contig_data() and buffer_skip()
provide easy methods to extract/insert data from/into a buffer.

buffer_write() and buffer_write_chunk() currently do not check
max_len nor to_forward, so they will quickly become embarrassing
to use or will need an equivalent. The reason is that they are
used to build error messages which currently are not subject to
analysis.