Generalize extra-* settings

This removes the extra-substituters and extra-sandbox-paths settings
and instead makes every array setting extensible by setting
"extra-<name> = <value>" in the configuration file or passing
"--<name> <value>" on the command line.

src/libstore/globals.hh

@@ -25,7 +25,7 @@ struct MaxBuildJobsSetting : public BaseSetting<unsigned int>
         options->addSetting(this);
     }
 
-    void set(const std::string & str) override;
+    void set(const std::string & str, bool append = false) override;
 };
 
 class Settings : public Config {
@@ -413,14 +413,6 @@ public:
     Setting<bool> sandboxFallback{this, true, "sandbox-fallback",
         "Whether to disable sandboxing when the kernel doesn't allow it."};
 
-    Setting<PathSet> extraSandboxPaths{
-        this, {}, "extra-sandbox-paths",
-        R"(
-          A list of additional paths appended to `sandbox-paths`. Useful if
-          you want to extend its default value.
-        )",
-        {"build-extra-chroot-dirs", "build-extra-sandbox-paths"}};
-
     Setting<size_t> buildRepeat{
         this, 0, "repeat",
         R"(
@@ -599,17 +591,6 @@ public:
         )",
         {"binary-caches"}};
 
-    // FIXME: provide a way to add to option values.
-    Setting<Strings> extraSubstituters{
-        this, {}, "extra-substituters",
-        R"(
-          Additional binary caches appended to those specified in
-          `substituters`. When used by unprivileged users, untrusted
-          substituters (i.e. those not listed in `trusted-substituters`) are
-          silently ignored.
-        )",
-        {"extra-binary-caches"}};
-
     Setting<StringSet> trustedSubstituters{
         this, {}, "trusted-substituters",
         R"(