Busybox should only multiplex based on argv[1] when it's called as "busybox".

Busybox should only multiplex based on argv[1] when it's called as "busybox".
Otherwise if you build busybox without a given applet you get the wrong error
message when you call it via a symlink to that applet.

(You also get the wrong behavior; it tries to use argv[1] as the command
name just like busybox does for _any_ unknown, and although I doubt
"echo rm -rf *" is common usage there's no upside and enough downside to
make me nervous.)

This fixes it.