sh(1) exec redirection

[Edgar Fuß]

After digging for hours why something like
        FOO_PASS_FD=3 exec /usr/pkg/bin/foo 3</path/to/pass
worked on -6, but not on -8, I found out that redir.c:1.38 introduced 
setting close-on-exec for redirections, 1.40 tried to fix it, but probably 
not for the command being exec'ed being exec.
At least 1.55 made it conditional on !posix, so I have a workaround.