* feat: add support for systemd socket activation
If webhook has been launched via systemd socket activation, simply use the systemd-provided socket rather than opening our own.
* docs: documentation for the systemd socket activation mode
* refactor: moved setuid and setgid flags into platform-specific section
The setuid and setgid flags do not work on Windows, so moved them to platform_unix so they are only added to the flag set on compatible platforms.
Also disallow the use of setuid and setgid in combination with -socket, since a setuid webhook process would not be able to clean up a socket that was created while running as root. If you _need_ to have the socket owned by root but the webhook process running as a normal user, you can achieve the same effect with systemd socket activation.
