Follow XDG specification for ~/.vpnns-NAME

[bigodel]

This is a very simple request, and should be pretty easy to implement, but could vpnns use the XDG Base Directory Specification for where it puts its state files? I think it would be as simple as replacing getenv("HOME") in

https://github.com/cernekee/ocproxy/blob/c98f06d942970cdf35dd66ab46840f7d6d567b60/src/vpnns.c#L842

with

	char* state_home = getenv("XDG_STATE_HOME");
	if (!state_home || strlen(state_home) == 0) {
		state_home = getenv("HOME");
		strncat(state_home, "/.local/state",
			sizeof(state_home) - strlen(state_home) - 1);
	}
	if (asprintf(&statedir, "%s/.vpnns-%s", getenv("HOME"), name) < 0)

Note that this might be improved as I haven't really done any serious projects in C before, but this takes into account the cases where the variable is set to the empty string as well.