Add support for named buffers. If you don't name a buffer, things work

Add support for named buffers. If you don't name a buffer, things work
much as before - buffers are automatically named "buffer0000",
"buffer0001" and so on and ordered as a stack. Buffers can be named
explicitly when creating ("loadb -b foo" etc) or renamed ("setb -b
buffer0000 -n foo"). If buffers are named explicitly, they are not
deleted when buffer-limit is reached. Diff from J Raynor.