disallow -f and -r being used together.

one can't send as fast as possible and with a random
delay simultaneously and this results in the following
crash reported on the list:

acid: stk()
nrand(n=0x0)+0x18 /sys/src/libc/port/nrand.c:13
sender(msglen=0x40,n=0x20,interval=0x0,fd=0x6)+0x151 /sys/src/cmd/ip/ping.c:268
main(argv=0xdfffefa4,argc=0x1)+0x212 /sys/src/cmd/ip/ping.c:571
_main+0x31 /sys/src/libc/386/main9.s:16

Reference: /n/sources/patch/sorry/ping-rf
Date: Mon Mar  9 14:29:10 CET 2009
Signed-off-by: quanstro@quanstro.net

--- /sys/src/cmd/ip/ping.c	Mon Mar  9 14:24:42 2009
+++ /sys/src/cmd/ip/ping.c	Mon Mar  9 14:24:41 2009
@@ -532,6 +532,10 @@
 		break;
 	} ARGEND;
 
+	if(flood && rint){
+		fprint(2, "-f and -r are not compatable\n");
+		usage();
+	}
 	if(msglen < proto->iphdrsz + ICMP_HDRSIZE)
 		msglen = proto->iphdrsz + ICMP_HDRSIZE;
 	if(msglen < 64)
--- /sys/man/8/ping	Mon Mar  9 14:24:49 2009
+++ /sys/man/8/ping	Mon Mar  9 14:24:48 2009
@@ -83,7 +83,9 @@
 adds the IP source and destination addresses to each report.
 .TP
 .B f
-send messages as fast as possible (flood).
+send messages as fast as possible (flood). This flag
+is not compatable with
+.BR -r .
 .TP
 .B i
 sets the time between messages
@@ -104,7 +106,9 @@
 .TP
 .B r
 randomizes the delay with a minimum extra delay of 0 ms and a
-maximum extra delay of the selected interval.
+maximum extra delay of the selected interval. This flag
+is not compatable with
+.BR -f .
 .TP
 .B s
 sets the length of the message to be