Existing use of the $ENV{TAIL} relied on parsing --std{out,err},
which was only usable for read-only daemons. However, -watch
doesn't use PublicInbox::Daemon code(*), so attempt to figure
out redirects.
(*) -watch won't able to run as a daemon in cases when
git-credential prompts for IMAP/NNTP passwords.
PublicInbox::Daemon is also designed for read-only
parallelism where all worker processes are the same.
Any subprocesses spawned by -watch are to do specific
tasks for a particular set of inboxes.