/etc/rc add trace debug and verify

Debugging boot issues can be helped by
logging each rc.d script as it is run
and being able to selectively enable/disable set -x
debug.sh provides an elaborate framework for debugging shell scripts.

For secure systems, we want to be paranoid about what we read
during boot.

dot() simply reads (.) arg file if it exists
vdot() if mac_veriexec is active, ignore unverified files
otherwise behaves much the same as dot()
safe_dot() in safe_eval.sh allows reading an untrusted file;
limiting the input to simple variable assignments.

In load_rc_config allow caller to provide an option to indicate how to
handle its arg:
-v use vdot()
-s use sdot() which will try to use vdot() and fallback to safe_dot()
The default is to read using dot()

rc_run_scripts()
encapsulate the running of rc.d scripts
so that we can easily call it more than twice.

We vdot local.rc.subr to pick up extensions (like
run_rc_scripts_final) and overrides.

We also allow rc.subr.local or rc.conf to set rc_config_xtra
eg (rc_config_xtra=XXX for historic compatibility)

rc use set -o verify around the reading in of rc.subr
This has no effect if mac_veriexec is not active, but if it is; ensures
rc.subr has not been tampered with.

Reviewed by: imp
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D43671