Split off device-specific stuff out of subr_autconf.c, and split off
autoconf-specific stuff out of device.h.

The only functional change is the removal of the unused evcnt.h include in
device.h which (*sigh*) has side-effects in x86's intr.h, and probably some
other in the rest of the tree but I'm only compiling i386's QEMU for the
time being.
Allow device_private(NULL), returning NULL.  It makes a lot of stuff
simpler, and in the situations where device_private was expected to crash,
it will soon enough anyway.
constify disk->dk_name.
Compile subr_device.c, too.