lei: TSTP affects all curl and related subprocesses
By relying more on pgroups for remaining remaining processes,
this lets us pause all curl+tail subprocesses with a single
kill(2) to avoid cluttering stderr.
We won't bother pausing the pigz/gzip/bzip2/xz compressor
process not cat-file processes, though, since those don't write
to the terminal and they idle soon after the workers react to
SIGSTOP.
AutoReap is hoisted out from TestCommon.pm. CLONE_SKIP
is gone since we won't be using Perl threads any time
soon (they're discouraged by the maintainers of Perl).