Split OptionalPathSetting from PathSetting

Rather than doing `allowEmpty` as boolean, have separate types and use
`std::optional`. This makes it harder to forget the possibility of an
empty path.

The `build-hook` setting was categorized as a `PathSetting`, but
actually it was split into arguments. No good! Now, it is
`Setting<Strings>` which actually reflects what it means and how it is
used.

Because of the subtyping, we now also have support for
`Setting<std::optional<String>>` in general. I imagine this can be used
to clean up many more settings also.

src/libstore/globals.hh

@@ -236,7 +236,7 @@ public:
         )",
         {"build-timeout"}};
 
-    PathSetting buildHook{this, true, "", "build-hook",
+    Setting<Strings> buildHook{this, {}, "build-hook",
         R"(
           The path to the helper program that executes remote builds.
 
@@ -556,8 +556,8 @@ public:
           line.
         )"};
 
-    PathSetting diffHook{
-        this, true, "", "diff-hook",
+    OptionalPathSetting diffHook{
+        this, std::nullopt, "diff-hook",
         R"(
           Absolute path to an executable capable of diffing build
           results. The hook is executed if `run-diff-hook` is true, and the