Fix "docker load" help by removing "SOURCE" prompt and mentioning STDIN

Fixes #3383
Cleanup: runtime.getFullName is deprecated by getFullName
Merge pull request #3384 from tianon/fix-load-help

Fix "docker load" help by removing "SOURCE" prompt and mentioning STDIN
Merge pull request #3420 from RoelVanNyen/getfullname

Cleanup: runtime.getFullName is deprecated by getFullName
update commands.go
update docker.go
move to pkg