It seems that  login.conf is not supported by xdm.
NetBSD-3.1, xdm is NetBSD's native, i.e. XFree-4.4 from src/.
Is it really unsupported or how can I enable it?

0 ~>ldd /usr/X11R6/bin/xdm
/usr/X11R6/bin/xdm:
        -lXau.6 => /usr/X11R6/lib/libXau.so.6
        -lXdmcp.6 => /usr/X11R6/lib/libXdmcp.so.6
        -lX11.6 => /usr/X11R6/lib/libX11.so.6
        -lXext.6 => /usr/X11R6/lib/libXext.so.6
        -lXinerama.1 => /usr/X11R6/lib/libXinerama.so.1
        -lXpm.4 => /usr/X11R6/lib/libXpm.so.4
        -lICE.6 => /usr/X11R6/lib/libICE.so.6
        -lSM.6 => /usr/X11R6/lib/libSM.so.6
        -lXt.6 => /usr/X11R6/lib/libXt.so.6
        -lXmu.6 => /usr/X11R6/lib/libXmu.so.6
        -lcrypt.0 => /usr/lib/libcrypt.so.0
        -lpam.0 => /usr/lib/libpam.so.0
        -lc.12 => /usr/lib/libc.so.12
0 ~>
-- 
Best regards, Aleksey Cheusov.