[MEDIUM] buffers: provide new buffer_feed*() function

[MEDIUM] buffers: provide new buffer_feed*() function

This functions act like their buffer_write*() counter-parts,
except that they're specifically designed to be used from a
stream interface handler, as they carefully check size limits
and automatically advance the read pointer depending on the
to_forward attribute.

buffer_feed_chunk() is an inline calling buffer_feed() as both
are the sames. For this reason, buffer_write_chunk() has also
been turned into an inline which calls buffer_write().