rmqa v0.1.2

Fixes from the second independent review.

• Files that aren't programs keep only their first 512 bytes, counted
  against the same 512 MiB budget as programs. In 0.1.1 each kept 4 KiB
  uncounted, so an archive of 150,000 one-byte files took about 680 MB of
  memory; it now takes about 70 MB (scripts/record-results.sh measures it).
• A cut-off archive is reported as damaged with the real reason, not as
  "programs too large", and a cut-off file under 4 KiB is no longer read as
  a short good one.
• A file whose path goes through a symlink stored earlier in the archive is
  unsafe-path, so a chain of links that each look harmless can't lead
  outside the folder.
• With --format github, every problem that stops the check is printed as
  an annotation, including a wrong number of folders.
• A program run with run that exits but leaves a child process holding
  its output is judged on what it printed.