libreboot-utils: unified error handling

i now use a singleton hook function per program:
nvmutil, mkhtemp and lottery

call this at the startup of your program:

(void) errhook(exit_cleanup);

then provide that function. make it static,
so that each program has its own version.

if you're writing a program that handles lots
of files for example, and you want to do certain
cleanup on exit (including error exit), this can
be quite useful.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/libreboot-utils/lib/usage.c

@@ -26,5 +26,5 @@ usage(void)
 	    util, util, util, util,
 	    util, util, util);
 
-	b0rk(EINVAL, "Too few arguments");
+	err_exit(EINVAL, "Too few arguments");
 }