devctl: move to using a uma zone

Convert the memory management of devctl. Rewrite if to make better
use of memory. This eliminates several mallocs (5? worse case) needed
to send a message. It's now possible to always send a message, though
if things are really backed up the oldest message will be dropped to
free up space for the newest.

Add a static bus_child_{location,pnpinfo}_sb to start migrating to
sbuf instead of buffer + length. Use it in the new code. Other code
will be converted later (bus_child_*_str is only used inside of
subr_bus.c, though implemented in ~100 places in the tree).

Reviewed by: markj@
Differential Revision: https://reviews.freebsd.org/D26140