debug.sh take advantage of hooks.sh

Allow Debug{On,Off} to run associated hooks.
This makes it feasible to collect extra debug info at runtime
without needing to modify any potentially verified and hence immutable
scripts.

Like debug.sh, hooks.sh is extremely handy and used extensively
in many parts of Junos and its build environment.
It is provided here in the hope that others may benefit.

We do not dot hooks.sh by default to reduce overhead, but it can
but done from rc.conf et al using

$_HOOKS_SH . /libexec/hooks.sh

to avoid including more than once.

Reviewed by: stevek
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D46552