sys: command: allow terminating the process early from log subroutine

sys: command: allow terminating the process early from log subroutine

If the logging subroutine $func returns CMD_FINISHED after processing a
line, the running subprocess is killed early.
This mechanism can be used when e.g. only a certain part of the output
of a (long-running) command is needed, avoiding the extra time it would
take the command to finish properly.

This is done in a entirely backwards-compatible way, i.e. existing
usages don't need any modification.

Signed-off-by: Christoph Heiss <c.heiss@proxmox.com>