veriexec: Add support in rc.d script processing.

[stevek]

Summary

Added dot function to include a file if it exists.
Added vdot function to do the same as dot, but verify the file first.

Also allow for including /etc/rc.subr.local, if it exists.

Obtained from: Juniper Networks, Inc.

[sjg]

I know a few folk are interested in using mac_veriexec, but we could push the guts of vdot etc into rc.subr.local (I've moved it to the end of the file) and just put a place holder here like vdot() { dot "$@"; }. It is the callouts in run_rc_* etc which are most important.

[sjg]

This is overcome by D43671