BUG/MEDIUM: lua: segfault with buffer_replace2

BUG/MEDIUM: lua: segfault with buffer_replace2

The function buffer_contig_space() returns the contiguous space avalaible
to add data (at the end of the input side) while the function
hlua_channel_send_yield() needs to insert data starting at p. Here we
introduce a new function bi_space_for_replace() which returns the amount
of space that can be inserted at the head of the input side with one of
the buffer_replace* functions.

This patch proposes a function that returns the space avalaible after buf->p.