load icewm Desktop via xdm

1) install icewm via pkg_add.
# pkg_add icewm

2) edit /etc/rc.conf.local for xdm at startup
xdm_flags=""

3) edit /etc/X11/xdm/Xsession, and replace the fvwm line to icewm.
/usr/local/bin/icewm

REFERENCE:
http://openbsdsupport.org/desktopOBSD.html