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.Config.preExecDelegate is just like
std.process.Config.preExecFunction, but can 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(
                preExecDelegate: () @trusted =>
                    prctl(PR_SET_PDEATHSIG, pdeathsig, 0, 0, 0) != -1,
            ),
        );
    }

preExecFunction is retained for backwards compatibility. If both
preExecFunction and preExecDelegate are given, both are called.