--psize option to randomize packet size

[yvs2014]

there's some error in signed/unsigned integer comparison which blocks packetsize randomizing:

$ sudo ./mtr --psize=-100 localhost
./mtr: invalid argument: '-100'

possible fix

--- a/ui/utils.c
+++ b/ui/utils.c
@@ -87 +87 @@ int strtonum_or_err(
-                if (num < INT_MAX)
+                if (((long)num) < INT_MAX)

https://github.com/yvs2014/mtr085/commit/c6a53de814bd8030381e9b7e30f6799ddf4963f4

then, it's good to limit possible values of 'psize' because

$ sudo ./mtr --psize=-28 localhost
floating point exception  sudo ./mtr --psize=-28 localhost

possible fix with

+            if (abs(ctl->cpacketsize) <= MINPACKET) {
+                ctl->cpacketsize = (ctl->cpacketsize > 0) ? (MINPACKET + 1) : -(MINPACKET + 1);
+            } else if (abs(ctl->cpacketsize) > MAXPACKET) {
+                ctl->cpacketsize = (ctl->cpacketsize > 0) ? MAXPACKET : -MAXPACKET;
+            }

https://github.com/yvs2014/mtr085/commit/34ea504a3bb2acc50560063dece653ee88e912b4