Add remain:yes option for run/task oneshot commands

Similar to systemd's RemainAfterExit=yes.  Prevents the task from
re-running on runlevel re-entry and ensures the post: script runs
when explicitly stopped or when leaving valid runlevels.

Useful for tasks that set up persistent state like firewall rules:

    task [2345] remain:yes \
         post:/usr/sbin/teardown-firewall \
         /usr/sbin/setup-firewall -- Firewall setup

Not supported for bootstrap-only tasks (runlevel S only) since these
are deleted immediately after completion.

man/finit.conf.5

@@ -398,6 +398,20 @@ mechanism completely, allows endless restarts.  Useful in many
 use-cases, but not what
 .Cm service
 was originally designed for so not the default behavior.
+.It Cm remain:yes
+for
+.Cm run
+and
+.Cm task
+only.  Prevents the task from re-running on runlevel re-entry and
+ensures the
+.Cm post:
+script runs when the task is explicitly stopped or leaves its valid
+runlevels.  Similar to systemd's
+.Cm RemainAfterExit=yes .
+.Pp
+.Sy Note:
+not supported for bootstrap-only tasks (runlevels [S] only).
 .It Cm oncrash:reboot
 when all retries have failed, and the service
 has