Don't segfault if run as non-root

Rather than segfaulting, print a message indicating that the binary
should be made setuid or some such.

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>
Close open file descriptors before exec'ing

So as not to leak file descriptors through to the program/user being
executed close open file descriptors above 2 (stderr).

Signed-off-by: Andrew Clayton <andrew@digital-domain.net>