Implement in-tree build for DCO with proto fix

The initialization hack for ovpn_tcp_prot doesn't work when the tcp_prot struct is constified (by Grsec/PaX, for instance) and is generally unsafe as it performs runtime function hooking/hijack to achieve its goal. The correct way to set up the structure requires access to symbols not exported by the kernel's headers, which makes out-of-tree compilation with the appropriate initializer impossible. In-tree builds can also benefit from LTO and other toolchain optimizations, as well as become part of monolithic kernels which do not use modules such as long-running network devices. Notes: Built and run using GCC 13 for Grsecurity/PaX (stable8) with all features except PRIVKSTACK and KERNSEAL enabled - x86_64 only. Built using LLVM17 with LTO & kCFI on GrapheneOS' linux-hardened patchset with their default features enabled - x86_64 only.