Resolve warnings on Windows

Appease the compiler by resolving some of the reported warnings,
including:

- Control paths that don't return.

- Potentially uninitialized variables.

- Unused local variables.

- Unreachable code.

- Type safety.

- Synchronize declarations with definitions for functions.

lib/roken/concat.c

@@ -100,7 +100,7 @@ roken_vmconcat (char **s, size_t max_len, va_list args)
 ROKEN_LIB_FUNCTION size_t ROKEN_LIB_CALL
 roken_mconcat (char **s, size_t max_len, ...)
 {
-    int ret;
+    size_t ret;
     va_list args;
 
     va_start(args, max_len);