MFC r306417: portsnap: only move expected snapshot contents from snap/ to files/

Previously it was possible to smuggle in addional files that would
be used by later portsnap runs. Now we only move those files expected
to be in the snapshot into files/ and require that there are no
unexpected files.

This was used by portsnap attacks 2, 3, and 4 in the "non-cryptanalytic
attacks against FreeBSD update components" anonymous gist.

MFC r305469:

Ensure that we always open only files that is named by explicitly
using shell redirections instead of having gzip(1) to decide what
file to open.

Issue reported in the "non-cryptanalytic attacks against freebsd
update components" anonymous gist.

Reviewed by: allanjude, emaste

MFC r264740:

Use case insensitive match in portsnap.

PR: bin/186510
Submitted by: olli

MFC r264740:

Use case insensitive match in portsnap.

PR: bin/186510
Submitted by: olli