Use /usr/bin/env to fix running it on FreeBSD

[subbink]

Currently the scripts use /usr/bin/perl to run the scripts, but on FreeBSD perl is not in /usr/bin, so using /usr/bin/env perl will let the scripts also work on FreeBSD.

[fanf2]

Thanks for this suggestion!

One of the things on my todo list is to publish nsdiff on CPAN with the usual perl packaging, so that it's easier to turn into system-specific packages. I'll try to make sure that this also fixes the shebang line on operating systems that don't put perl in the standard place.

I'll leave this pull request open as a reminder.