libpcap regression since 1.5.3

[Dario Bertini <berdario () gmail com>]

When running a SYN scan, a lot of probes are dropped, and this slows
down the scan terribly:

https://gist.github.com/berdario/9e2eecba0d35f9255f29

If I wait for the scan to finish, subsequent scans are faster
(probably until the network interface is torn down or anyhow until the
next reboot)

I reproduced the problem on Nixos, Ubuntu, Kali... and I recompiled
nmap with different versions of libpcap

1.5.3 and 1.7.2 (the latest) are affected
1.3.0 and 1.4.0 aren't

I can reproduce the problem while connecting to a machine on my
network, I haven't been able to reproduce the problem by scanning
localhost
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