sbuf(9): Add NOWAIT dynamic buffer extension mode

The goal is to avoid some kinds of low-memory deadlock when formatting
heap-allocated buffers.

Reviewed by: vangyzen
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D21015

(cherry picked from commit 71db411eb627601a5f9a3464f88d75741927ce60)