Move to using sbuf for some sysctl in newbus

Convert two different sysctl to using sbuf. First, for all the default
sysctls we implement for each device driver that's attached. This is a
pure sbuf conversion.

Second, convert sysctl_devices to fill its buffer with sbuf rather
than a hand-rolled crappy thing I wrote years ago.

Reviewed by: cem, markj
Differential Revision: https://reviews.freebsd.org/D26206