v0.1.4

The three findings left over from the original review, each with a regression
test. No interface change: same commands, same exit codes, same state names.

• logs no longer reads the whole file to print the end of it. logs -n 20
  slurped the entire log into memory first; on a multi-gigabyte training log —
  exactly what this tool exists to supervise — that is gigabytes to print twenty
  lines. It now reads backwards in 64 KiB blocks and stops as soon as it has the
  lines asked for. Streaming the whole log (logs with no -n) no longer buffers
  it either. Measured by a test that counts bytes read: ~4 MB file, 3 lines
  wanted, under 128 KiB read.
• A failed launch no longer destroys the previous run's record. The run
  directory was deleted before systemd-run was known to have succeeded, so a
  typo in the command threw away the logs and exit status of the run being
  replaced. The old directory is now moved aside and only discarded once the new
  unit is actually up; every failure path puts it back.
• Two concurrent proc run of the same name can no longer corrupt each
  other. Both passed the RUNNING check and the loser deleted the winner's run
  directory out from under a live unit. Launches are now serialized per name by
  an flocked lock file, giving one clean winner and one clean EX_BUSY.
• meta.json is written 0600. --env is the one path by which a real
  secret reaches that file, and it was world-readable. Created with the mode
  rather than chmoded afterwards, so there is no window. Note that --env is
  still not a secret channel — the values are unit properties and visible to your
  own user; the README says so plainly now.
• Fixed in passing: when a followed process died, logs -f read the bytes
  written in its last moments and discarded them instead of printing them.