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...)

conf.c

@@ -184,6 +184,7 @@ conf_init(struct conf *c)
 	c->flags = 0;
 	c->bwidth = CONF_BWIDTH;
 	c->mamount = CONF_MAMOUNT;
+	c->snapdist = CONF_SNAPDIST;
 
 	TAILQ_INIT(&c->ignoreq);
 	TAILQ_INIT(&c->cmdq);