device_printf: Use sbuf for more coherent prints on SMP

device_printf does multiple calls to printf allowing other console messages to
be inserted between the device name, and the rest of the message. This change
uses sbuf to compose to two into a single buffer, and prints it all at once.

It exposes an sbuf drain function (drain-to-printf) for common use.

Update documentation to match; some unit tests included.

Submitted by: jmg
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D16690