sbuf: Process more than one char at a time

Revamp sbuf_put_byte() to sbuf_put_bytes() in the obvious fashion and
fixup callers.

Add a thin shim around sbuf_put_bytes() with the old ABI to avoid ugly
changes to some callers.

Reviewed by: jhb, markj
Obtained from: Dan Sledz
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D3717