Add enumeration for sel.mode

This patch also prevents sel.mode from increasing beyond 2. It is almost
impossible, but sel.mode may overflow if mouse is moved around for too
long while selecting.

st.c

@@ -162,6 +162,12 @@ enum window_state {
 	WIN_FOCUSED = 2
 };
 
+enum selection_mode {
+	SEL_IDLE = 0,
+	SEL_EMPTY = 1,
+	SEL_READY = 2
+};
+
 enum selection_type {
 	SEL_REGULAR = 1,
 	SEL_RECTANGULAR = 2
@@ -643,7 +649,7 @@ void
 selinit(void) {
 	memset(&sel.tclick1, 0, sizeof(sel.tclick1));
 	memset(&sel.tclick2, 0, sizeof(sel.tclick2));
-	sel.mode = 0;
+	sel.mode = SEL_IDLE;
 	sel.ob.x = -1;
 	sel.primary = NULL;
 	sel.clipboard = NULL;
@@ -897,7 +903,7 @@ bpress(XEvent *e) {
 
 		/* Clear previous selection, logically and visually. */
 		selclear(NULL);
-		sel.mode = 1;
+		sel.mode = SEL_EMPTY;
 		sel.type = SEL_REGULAR;
 		sel.oe.x = sel.ob.x = x2col(e->xbutton.x);
 		sel.oe.y = sel.ob.y = y2row(e->xbutton.y);
@@ -920,7 +926,7 @@ bpress(XEvent *e) {
 		 * make clicks visible
 		 */
 		if(sel.snap != 0) {
-			sel.mode++;
+			sel.mode = SEL_READY;
 			tsetdirt(sel.nb.y, sel.ne.y);
 		}
 		sel.tclick2 = sel.tclick1;
@@ -1142,13 +1148,12 @@ brelease(XEvent *e) {
 	if(e->xbutton.button == Button2) {
 		selpaste(NULL);
 	} else if(e->xbutton.button == Button1) {
-		if(sel.mode < 2) {
-			selclear(NULL);
-		} else {
+		if(sel.mode == SEL_READY) {
 			getbuttoninfo(e);
 			selcopy(e->xbutton.time);
-		}
-		sel.mode = 0;
+		} else
+			selclear(NULL);
+		sel.mode = SEL_IDLE;
 		tsetdirt(sel.nb.y, sel.ne.y);
 	}
 }
@@ -1165,7 +1170,7 @@ bmotion(XEvent *e) {
 	if(!sel.mode)
 		return;
 
-	sel.mode++;
+	sel.mode = SEL_READY;
 	oldey = sel.oe.y;
 	oldex = sel.oe.x;
 	oldsby = sel.nb.y;