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@

cwmrc.5

@@ -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: November 27 2013 $
+.Dd $Mdocdate: December 13 2013 $
 .Dt CWMRC 5
 .Os
 .Sh NAME
@@ -341,12 +341,14 @@ Raise current window.
 Label current window.
 .It freeze
 Freeze current window geometry.
+.It fullscreen
+Full-screen current window (gap + border removed).
 .It maximize
-Maximize current window full-screen.
+Maximize current window (gap + border honored).
 .It vmaximize
-Maximize current window vertically.
+Vertically maximize current window (gap + border honored).
 .It hmaximize
-Maximize current window horizontally.
+Horizontally maximize current window (gap + border honored).
 .It moveup
 Move window
 .Ar moveamount