introduce a new config option to snap to the screen edge. 'snapdist'

keyword taken from a diff from Sviatoslav Chagaev to do the same thing,
but implemented in a completely way (based on some very old code from
mk@).  default set to 0, so no behavior change.

ok oga@ (who would also like to take it further...)

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: September 25 2010 $
+.Dd $Mdocdate: May 7 2011 $
 .Dt CWMRC 5
 .Os
 .Sh NAME
@@ -277,6 +277,8 @@ Reverse cycle through groups.
 Forward cycle through windows.
 .It rcycle
 Reverse cycle through windows.
+.It snapdist
+Minimum distance to snap-to adjacent edge.
 .It delete
 Delete current window.
 .It hide