Add prototype to &File::chmod::chmod

I've been running a small script using File::chmod::chmod() for a little over a year on OpenBSD with the system perl (5.12.2).  No problems noted.  Then, this week, I upgraded the OS, which upgrades system Perl to 5.16, and now the script generates the following warning:

Prototype mismatch: sub main::chmod (@) vs none

I installed a couple older perls with plenv and turned "use" statements on and off for a longish time and determined that it's the combination of Perl 5.14+, autodie, and File::chmod that causes the warning.  

This branch contains a change to &File::chmod::chmod (adds prototype (@) to sub definition) and adds the line "use autodie;" to each of the four tests to verify that the fix works.