Rename warn() to warning() to fix symbol conflicts on BSD and Mac OS

Rename warn() to warning() to fix symbol conflicts on BSD and Mac OS

This fixes a problem reported by Randal Schwartz:

>I finally tracked down all the (albeit inconsequential) errors I was getting
>on both OpenBSD and OSX.  It's the warn() function in usage.c.  There's
>warn(3) in BSD-style distros.  It'd take a "great rename" to change it, but if
>someone with better C skills than I have could do that, my linker and I would
>appreciate it.

It was annoying to me, too, when I was doing some mergetool testing on
Mac OS X, so here's a fix.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: "Randal L. Schwartz" <merlyn@stonehenge.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>