staslib: parse time spans the way systemd does

[martin-belanger]

dc-giveup-timeout is documented as a time span, and nvme-discoverd takes the same key with the same encoding, parsing it with a copy of systemd's parse_time(). A span a user writes once has to mean the same thing in both daemons. It did not: timeparse.py was a vendored copy of pytimeparse, which has a grammar of its own.

500ms systemd 0.5s pytimeparse rejected
1week systemd 604800s pytimeparse rejected
1y systemd 31557600s pytimeparse rejected
1:30 systemd rejected pytimeparse 90s

The parser now implements systemd's grammar, including the suffix table, the "M" months / "m" minutes case distinction, the 30.44-day month and 365.25-day year, and the truncating arithmetic on fractional digits. Nothing of pytimeparse remains, so the file drops the vendored MIT notice it no longer owes and takes the same header as the rest of staslib.

Spans only pytimeparse accepted now fall back to the default with a warning: "hrs"/"secs"/"mins"/"dy", uppercase, comma-separated terms, colon notation, and "inf"/"1e5"/"nan". One value keeps parsing and changes meaning: "1M" was 60 seconds and is now one month.

stafd.conf(5) and the sample stafd.conf now point at systemd.time(7). The tests mirror nvme-cli's, and every case was checked against the host's systemd-analyze timespan.

[codecov]

Codecov Report

❌ Patch coverage is 98.78049% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.32%. Comparing base (a3e2152) to head (fc93ae9).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
staslib/timeparse.py	98.78%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #517      +/-   ##
==========================================
+ Coverage   70.86%   71.32%   +0.46%     
==========================================
  Files          16       16              
  Lines        2742     2786      +44     
==========================================
+ Hits         1943     1987      +44     
  Misses        799      799              

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