Style fixes (nullptr)

Previously, Qrack relied on NULL when necessary for pointers. In this release, all instances of NULL have been replaced with nullptr (since this was added in C++11, as far back as Qrack supports for a C++ standard). This surfaced exactly one instance of NULL where nullptr was not immediately appropriate, because NULL was assigned to a variable for an instance of a class rather than a pointer to the class. Deeming this a "code smell" (even though it's very unlikely there could have been any bad access, use-after-free, or other memory safety error, here) the code was refactored to achieve its same original intent while using nullptr instead. Also, for loops have been brought up to standard for use of the "foreach" pattern (for (auto& x : container)).

Full Changelog: vm6502q.v9.13.33...vm6502q.v9.13.37

sha1sum results:
41f83401b38385ed523a17d397aaae200ad93f7b libqrack-macosx_14_0_arm64.zip
4a33ab54b167fe4efd5dd9de17787dcadecc323b libqrack-macosx_15_0_arm64.zip
9f2d1fbf712136223dbe158d88dd643b0390c2de libqrack-manylinux2014_x86_64.zip
cb2ed1a9740477b7efc7b2926b916df557841a5a libqrack-manylinux_2_35_x86_64.zip
8fc673d3d14e28a6930b2dfb4a7fbf1cca1b4848 libqrack-manylinux_2_39_x86_64.zip
5d6cc4b8f7509ef90446d355d97a5e30ac9ee603 libqrack-win-amd64.zip