[all] use explicit void parameter lists

This makes it a compile-time error to call a function that semantically
takes no parameters with a nonzero number of arguments.

Previously, such code would still compile, but risk blowing up the stack
if a compiler chose to use something other than caller-cleanup calling
conventions.

module/test.c

@@ -11,7 +11,7 @@
 
 #include "kvmfr.h"
 
-int main()
+int main(void)
 {
   int fd = open("/dev/kvmfr0", O_RDWR);
   if (fd < 0)