improve MTU probe performance

[Peng Haibo]

Hi all

I try to use tinc within my network, but I find almost all mtu probe take
30 times to fix, it's never really fix at all.

after read code,  i think the problem case by function receive_udppacket();
and line :
origin -= MTU / 64 + 20;

here , MTU / 64 + 20 =43, this means, every time we send a mtu probe
packet, we get a reply smaller then what we send.

review the commit log, the line added for keep saft to compress.

for capable with current version, I can't remove this line, so I add 43
back then we got reply. and once MTU fix, apply the saft margine when set
mtu  "n->mtu = n->minmtu - MTU / 64 - 20;"

now, I can fix MTU within about 10 probes.

2020-02-13 23:20:05 tinc.firma[28376]: Fixing MTU of office01
(123.120.166.214 port 655) to 1408 after 6 probes
2020-02-13 23:20:08 tinc.firma[28376]: Fixing MTU of vmub1804
(111.196.162.156 port 59172) to 1405 after 7 probes
2020-02-13 23:20:08 tinc.firma[28376]: Fixing MTU of home02
(111.196.162.156 port 52283) to 1403 after 7 probes
2020-02-13 23:20:09 tinc.firma[28376]: Fixing MTU of home01
(111.196.162.156 port 655) to 1403 after 8 probes
2020-02-13 23:20:30 tinc.firma[28376]: Fixing MTU of openwrt01
(106.121.163.249 port 48968) to 1363 after 30 probes
2020-02-13 23:21:17 tinc.firma[28376]: Fixing MTU of office01
(123.120.166.214 port 655) to 1406 after 8 probes
2020-02-13 23:22:29 tinc.firma[28376]: Fixing MTU of office01
(123.120.166.214 port 655) to 1403 after 8 probes
2020-02-13 23:25:19 tinc.firma[28376]: Fixing MTU of openwrt01
(106.121.163.249 port 48968) to 1365 after 30 probes
2020-02-13 23:29:03 tinc.firma[28376]: Fixing MTU of openwrt01
(106.121.163.249 port 48968) to 1364 after 30 probes
2020-02-13 23:29:57 tinc.firma[28376]: Fixing MTU of vmub1804
(111.196.162.156 port 61655) to 1404 after 5 probes
2020-02-13 23:40:47 tinc.firma[28376]: Fixing MTU of home02
(111.196.162.156 port 52283) to 1403 after 5 probes
2020-02-13 23:54:16 tinc.firma[28376]: Fixing MTU of home01
(111.196.162.156 port 655) to 1403 after 8 probes
2020-02-14 00:07:27 tinc.firma[28376]: Fixing MTU of openwrt01
(106.121.163.249 port 48968) to 1365 after 30 probes

client openwrt01 use version 1.0.35, still try to probe mtu and ervery time
take 30time.

also I try to reduce network usage during probe which alreay done in
1.1.pre17

the patch attached base on 1.0.36

thanks
Haibo Peng
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