daemon/shell: refactor redirection of 0/1/2 from /dev/null

daemon/shell: refactor redirection of 0/1/2 from /dev/null

Both daemon.c and shell.c contain logic to open FDs 0/1/2 from
/dev/null if they are not already open.  Move the function in daemon.c
to setup.c and use it in shell.c, too.

While there, remove a 'not' that inverted the meaning of the comment.
The point is indeed to *avoid* messing up.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>