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