portsnap: use lam on the known good hash list

This change is equivalent to the approach committed in r306417, but if
sed has a bug it could be exploited by the untrusted tar file. Instead,
generate the expected tar content and compare that with find's output.

convert the expected hash list to the expected tar content filesystem
layout, and compare that with find's output.

Submitted by: cperciva (in review D8052)
Reviewed by: oshogbo
MFC after: 2 weeks