Report exactly one email per run and none for an empty run

[zaihuaji]

Summary

• The progress report is parked in dscheck.einfo instead of being sent, so it goes out exactly when PBS kills the job off its walltime - in the same daemon pass that resubmits the check. The final report sends directly and drops the parked one: left behind it would be mailed a second time as a stale report and would hold the finished check back from being purged.
• A forked child could reach quit_dsquasar() and report for the whole run, emailing a duplicate, clearing the parent's parked report and marking the shared check record failed while the parent was still working. Only the parent reports now.
• The report is dropped when nothing was processed and nothing failed, for every backup action rather than the transfer one alone. A parked progress report still forces a final report, to clean it up.
• Version bumped to 3.0.13.

Test plan

• 5 files -> 1 mail; 0 files with no error -> 0 mails; 0 files with an error count -> 1 mail; 0 files with a parked report -> 1 mail
• -A 16 statistics and -A 8 check still report with 0 backup files; no -e/-E -> 0 mails; no dscheck -> 1 direct mail
• Parent quit -> 1 mail; child quit -> 0 mails and sys.exit(1)
• Deadline check called twice -> exactly 1 park; park then finish -> 1 mail and einfo cleared