Show usage if run without arguments
Show usage if run without arguments

Before, running `./miri` without arguments gave
'./miri: 92: shift: can't shift that many' if run with /bin/sh,
or no output at all when run with bash.

Although that gives some indication of the error, it's not as helpful as
showing the usage.

rust-lang/miri,rust-lang/miri,rust-lang/miri,tsion/miri,rust-lang/miri