Portability: /bin/sh is present on every *X OS. (Well, SCO Unix and an old HP-UX might need the workaround ${1+"$@"} for a historic bug with "$@".)
It works with any compatible shell, too.
I am not sure what your version of nohup is doing.
Try the following nohup.sh script:
#!/bin/sh
# ignore signals HUP INT QUIT
trap "" 1 2 3
# redirect filedescriptors 0...6
exec "$@" </dev/null...