MEDIUM: stream-int: replace channel_alloc_buffer() with si_alloc_ibuf() everywhere

Well that's only 3 places (applet.c, stream_interface.c, hlua.c). This
ensures we always clear SI_FL_WAIT_ROOM before setting it on failure,
so that it is granted that SI_FL_WAIT_ROOM always indicates a lack of
room for doing an operation, including the inability to allocate a
buffer for this.
3 files changed