Add drain functionality to sbufs. The drain is a function that is
called when the sbuf internal buffer is filled. For kernel sbufs with a
drain, the internal buffer will never be expanded. For userland sbufs
with a drain, the internal buffer may still be expanded by
sbuf_[v]printf(3).

Sbufs now have three basic uses:

1. static string manipulation. Overflow is marked.
2. dynamic string manipulation. Overflow triggers string growth.
3. drained string manipulation. Overflow triggers draining.

In all cases the manipulation is 'safe' in that overflow is detected and
managed.

Reviewed by: phk (the previous version)