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

src/basic/path-util.h

@@ -102,3 +102,6 @@ char *prefix_root(const char *root, const char *path);
 int parse_path_argument_and_warn(const char *path, bool suppress_root, char **arg);
 
 char* dirname_malloc(const char *path);
+
+bool filename_is_valid(const char *p) _pure_;
+bool path_is_safe(const char *p) _pure_;