alloc_buffer: Return unqualified pointer type in alloc_buffer_next

alloc_buffer: Return unqualified pointer type in alloc_buffer_next

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.