libdevinfo: Avoid false positives for the root0 sentinel value

Previously, a NULL pointer value was used to request the root0 device
at the top of the device tree. However, this meant that resource
ranges from a rman with a NULL device pointer were annotated as being
owned by root0 instead of being unowned. Switch to a different value
for root0's sentinel to avoid the clash.

Since this is an ABI change, bump the SHLIB_MAJOR for libdevinfo to 7.

Reported by: jrtc27
Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D48675