Implement most ICCCM rules for selection handling.

ICCCM mandates the use of real timestamps to interact with the
selection, to rule out race conditions if the clients are run at
different speeds. I have implemented the low hanging fruit, putting the
timestamps into text selection. Also, ICCCM mandates a check for whether
XSetSelectionOwner() worked. Not sure my version is correct, though.

st.c

@@ -423,7 +423,7 @@ static void xsettitle(char *);
 static void xresettitle(void);
 static void xsetpointermotion(int);
 static void xseturgency(int);
-static void xsetsel(char *);
+static void xsetsel(char *, Time);
 static void xtermclear(int, int, int, int);
 static void xunloadfont(Font *);
 static void xunloadfonts(void);
@@ -449,7 +449,7 @@ static void selinit(void);
 static void selnormalize(void);
 static inline bool selected(int, int);
 static char *getsel(void);
-static void selcopy(void);
+static void selcopy(Time);
 static void selscroll(int, int);
 static void selsnap(int, int *, int *, int);
 static int x2col(int);
@@ -984,8 +984,8 @@ getsel(void) {
 }
 
 void
-selcopy(void) {
-	xsetsel(getsel());
+selcopy(Time t) {
+	xsetsel(getsel(), t);
 }
 
 void
@@ -997,7 +997,7 @@ selnotify(XEvent *e) {
 	XSelectionEvent *xsev;
 
 	ofs = 0;
-	xsev = (XSelectionEvent *)e;
+	xsev = &e->xselection;
 	if (xsev->property == None)
 	    return;
 	do {
@@ -1083,6 +1083,9 @@ selrequest(XEvent *e) {
 	xev.selection = xsre->selection;
 	xev.target = xsre->target;
 	xev.time = xsre->time;
+        if (xsre->property == None)
+            xsre->property = xsre->target;
+
 	/* reject */
 	xev.property = None;
 
@@ -1125,11 +1128,13 @@ selrequest(XEvent *e) {
 }
 
 void
-xsetsel(char *str) {
+xsetsel(char *str, Time t) {
 	free(sel.primary);
 	sel.primary = str;
 
-	XSetSelectionOwner(xw.dpy, XA_PRIMARY, xw.win, CurrentTime);
+	XSetSelectionOwner(xw.dpy, XA_PRIMARY, xw.win, t);
+        if (XGetSelectionOwner(xw.dpy, XA_PRIMARY) != xw.win)
+            selclear(0);
 }
 
 void
@@ -1146,7 +1151,7 @@ brelease(XEvent *e) {
 			selclear(NULL);
 		} else {
 			getbuttoninfo(e);
-			selcopy();
+			selcopy(e->xbutton.time);
 		}
 		sel.mode = 0;
 		tsetdirt(sel.nb.y, sel.ne.y);