Rename "policy" to "trust"

[aweiteka]

The word "trust" seems to resonate with docker users. See Docker Content Trust. The skopeo policy file describes trust quite well but its use must be translated for the uninitiated.

I'm not suggesting we do a global search and replace, but in cases where users interact with files or output, "trust" is a better term to use IMO. Specifically, these files:

cmd/skopeo/copy.go
cmd/skopeo/main.go
docs/skopeo.1.md
Makefile

cc @rhatdan

[rhatdan]

Make sense to me.

[runcom]

+1

[rhatdan]

#192