tools: Only use our private path when looking for external commands

Instead of looking for external commands in the path, just look
for them in our private directory.

We want to be conservative early on, and limit what sorta things
we have to maintain later. We can later remove this restriction
if a real use case presents itself.
tools: Fix passing args to external commands

There were various bugs passing arguments, with duplicates being
passed, as well as certain arguments being skipped.t
trust: Print out usage when extract-trust run incorrectly

Also sorta covers --help and -h usage
trust: Move the extract-trust external placeholder command into trust/