[OPTIM] buffer: replace rlim by max_len

[OPTIM] buffer: replace rlim by max_len

In the buffers, the read limit used to leave some place for header
rewriting was set by a pointer to the end of the buffer. Not only
this required subtracts at every place in the code, but this will
also soon not be usable anymore when we want to support keepalive.

Let's replace this with a length limit, comparable to the buffer's
length. This has also sightly reduced the code size.