Implement support for EWMH's _NET_WM_STATE_FULLSCREEN hint.

Since we already have a form of 'maximize', we need to differentiate
between 'maximize' and the new 'fullscreen' mode.  The 'maximize' mode
will continue to honor gap but now *retains* the border, matching the
'vert/horz maximize' behaviour.  The new 'fullscreen' mode supports and
follows the _NET_WM_STATE_FULLSCREEN hint, allowing the client perform
additional window modifications; in this mode, cwm(1) will *ignore* gap,
remove borders and freeze(move/resize) the client.  Additionally,
'fullscreen' mode will remember various combinations of previous states.

* default keybinding changes: CM-f 'fullscreen', CM-m 'maximize' (re-map
  as desired).

Positive feedback from a few, testing and ok sthen@

cwm.1

@@ -14,7 +14,7 @@
 .\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 .\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 .\"
-.Dd $Mdocdate: May 9 2012 $
+.Dd $Mdocdate: October 31 2012 $
 .Dt CWM 1
 .Os
 .Sh NAME
@@ -91,7 +91,9 @@ Reverse cycle through active groups.
 .It Ic CMS-f
 Toggle freezing geometry of current window.
 .It Ic CM-f
-Toggle full-screen size of current window.
+Toggle full-screen mode of current window.
+.It Ic CM-m
+Toggle maximization of current window.
 .It Ic CM-=
 Toggle vertical maximization of current window.
 .It Ic CMS-=