libdtrace: Add kinst support

kinst does not instantiate its probes automatically, it only does so on
demand via an ioctl interface implemented by /dev/kinst. This change
modifies libdtrace to perform that work when the script references the
kinst provider, similar to the way pid provider probes are implemented.

Reviewed by: markj
MFC after: 3 months
Sponsored by: Google, Inc. (GSoC 2022)
Differential Revision: https://reviews.freebsd.org/D36852