enhanced path sanitizing

src/str.h

@@ -35,4 +35,4 @@ char *mg_hex(const void *buf, size_t len, char *dst);
 void mg_unhex(const char *buf, size_t len, unsigned char *to);
 unsigned long mg_unhexn(const char *s, size_t len);
 int mg_check_ip_acl(struct mg_str acl, uint32_t remote_ip);
-char *mg_remove_double_dots(char *s);
+bool mg_path_is_sane(const char *path);