util: move filename_is_valid() and path_is_safe() to path-util.[ch]

src/test/test-util.c

@@ -50,6 +50,7 @@
 #include "user-util.h"
 #include "util.h"
 #include "virt.h"
+#include "path-util.h"
 
 static void test_streq_ptr(void) {
         assert_se(streq_ptr(NULL, NULL));