alloc_buffer_next is useful for peeking to the remaining part of the
buffer and update it, with subsequent allocation (once the length
is known) using alloc_buffer_alloc_bytes. This is not as robust
as the other interfaces, but it allows using alloc_buffer with
string-writing interfaces such as snprintf and ns_name_ntop.
This commit adds fixed-size allocation buffers. The primary use
case is in NSS modules, where dynamically sized data is stored
in a fixed-size buffer provided by the caller.
Other uses include a replacement of mempcpy cascades (which is
safer due to the size checking inherent to allocation buffers).
