appc: fix relative paths in app validator

strings.TrimPrefix was not correct since it resulted in different
behaviour depending on whether the directory passed had a trailing slash
or not (this meant for example that `actool validate foo/` would succeed
but `actool validate foo` would fail)
Go has a function for the correct behaviour in filepath.Rel, so use it.

Also small style updates to keep up with the recent
appmanifest+filesetmanifest --> ImageManifest change
Merge pull request #249 from jonboulle/fix

appc: fix relative paths in app validator