Promote std.process.Config.preExecFunction to a delegate

std.process.Config.preExecFunction is now a delegate instead of a function
pointer, and can therefore capture an environment, for example:

    import core.sys.linux.sys.prctl : PR_SET_PDEATHSIG, prctl;
    import std.process : Config, execute;

    void runProgram(int pdeathsig)
    {
        execute(
            ["program"],
            config: Config(
                preExecFunction: () @trusted =>
                    prctl(PR_SET_PDEATHSIG, pdeathsig, 0, 0, 0) != -1,
            ),
        );
    }

Despite function pointers implicitly converting to delegates, this is a
backwards-incompatible change, as user code may rely on the field being a
function pointer. For example, code like the following will no longer compile:

    import std.process : Config;

    nothrow pure @nogc @safe
    bool f() { return true; }

    void example()
    {
        auto config = Config(preExecFunction: &f);
        bool function() g = config.preExecFunction;
    }

changelog/std.process.Config.preExecFunction-delegate.dd

@@ -0,0 +1,38 @@
+Promote `std.process.Config.preExecFunction` to a delegate
+
+$(LINK2 $(ROOT_DIR)phobos/std_process.html#.Config.preExecFunction,
+`std.process.Config.preExecFunction`) is now a delegate instead of a function
+pointer, and can therefore capture an environment, for example:
+
+-------
+import core.sys.linux.sys.prctl : PR_SET_PDEATHSIG, prctl;
+import std.process : Config, execute;
+
+void runProgram(int pdeathsig)
+{
+    execute(
+        ["program"],
+        config: Config(
+            preExecFunction: () @trusted =>
+                prctl(PR_SET_PDEATHSIG, pdeathsig, 0, 0, 0) != -1,
+        ),
+    );
+}
+-------
+
+Despite function pointers implicitly converting to delegates, this is a
+backwards-incompatible change, as user code may rely on the field being a
+function pointer. For example, code like the following will no longer compile:
+
+-------
+import std.process : Config;
+
+nothrow pure @nogc @safe
+bool f() { return true; }
+
+void example()
+{
+    auto config = Config(preExecFunction: &f);
+    bool function() g = config.preExecFunction;
+}
+-------

std/process.d

@@ -1271,6 +1271,22 @@ version (Posix)
     assert(received);
 }
 
+version (Posix)
+@safe unittest
+{
+    foreach (i; 0 .. 3)
+    {
+        auto config = Config(
+            preExecFunction: delegate() @trusted {
+                _Exit(i);
+                return true;
+            },
+        );
+        auto pid = spawnProcess(["false"], config: config);
+        assert(wait(pid) == i);
+    }
+}
+
 /*
 Implementation of spawnProcess() for Windows.
 
@@ -2188,11 +2204,11 @@ struct Config
 
         On Windows, this member is not available.
         */
-        bool function() nothrow @nogc @safe preExecFunction;
+        bool delegate() nothrow @nogc @safe preExecFunction;
     }
     else version (Posix)
     {
-        bool function() nothrow @nogc @safe preExecFunction;
+        bool delegate() nothrow @nogc @safe preExecFunction;
     }
 }