dev-cura: Make into a proper script

dev-cura: Make into a proper script

Firstly, move the #! to the top, as the kernels require (otherwise if
you are lucky your shell can run it, but wrapper programs can't).

Secondly, use `exec' to have dev-cura exec cura without forking.

With these two changes, it is easier to strace dev-cura: you can run
  strace -ot ./dev-cura
rather than
  strace -fot bash ./dev-cura

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>