Add a tool to verify runtimes against a mtree manifest

This is something I've been thinking about adding for a while.

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• tests/valgrind.sh: Don't show reachable objects by default

  There are too many false positives if we do.

• populate-depot: Only put permissions in mode=, not all of st_mode

  This matches the output of bsdtar (libarchive), for example:

    bsdtar -cf- --format=mtree -C ~/tmp .

  and NetBSD mtree(8) (the mtree-netbsd package in Debian), for example:

    mtree -c -p ~/tmp | mtree -C

• populate-depot: Write a top-level mtree

  This provides a convenient way to check whether the depot has the contents we believe it should.

• populate-depot: Avoid excess precision for modification times

• pv-mtree: Factor out parsing and applying the mtree manifest

  This will allow a similar code path that verifies instead of applying.

• pv-verify: New utility to verify SteamLinuxRuntime_foo against a mtree

  This will make it easy to verify that SteamLinuxRuntime_sniper and SteamLinuxRuntime_soldier are intact:

    .../SteamLinuxRuntime_sniper/pressure-vessel/bin/pv-verify

  and only slightly more difficult to do the same for SLR 1.0:

    .../SteamLinuxRuntime_soldier/pressure-vessel/bin/pv-verify \
    .../SteamLinuxRuntime

  A more explicit version of the first command is:

    .../SteamLinuxRuntime_sniper/pressure-vessel/bin/pv-verify \
        --mtree=.../SteamLinuxRuntime_sniper/mtree.txt.gz \
        -- \
        .../SteamLinuxRuntime_sniper/pressure-vessel

  but the defaults are sensible.

• tests: Test pv-verify