[sandbox] Landlock support

* Support disabling Landlock at compile time or runtime if unsupported,
  without disabling seccomp
* Support older Landlock ABI versions
* Support an extra arbitrary RW path, inteded for the coredump dir
* Support database locations other than the working directory

src/settings.hpp

@@ -5,6 +5,7 @@
 namespace settings {
     extern int VERBOSITY;
     extern bool SANDBOX;
+    extern std::string SANDBOXEXTRAPATH;
     extern int LOGINPORT;
     extern bool APPROVEALLNAMES;
     extern bool AUTOCREATEACCOUNTS;