clean exit from tcpdump with asan

[enh via tcpdump-workers <tcpdump-workers () lists tcpdump org>]

> --- Begin Message ---
>
>
> From: enh <enh () google com>
>
> Date: Wed, 14 Oct 2020 14:28:09 -0700
>
> i haven't reproduced it myself yet (though i'll try shortly) but we
> got an automated crash report from tcpdump on Android via
> [gwp-asan](https://developer.android.com/ndk/guides/gwp-asan).
>
> the bug is a use-after-free, specifically when pcap_breakloop() tries
> to write to the already-freed struct pcap_t. this happens if a signal
> is received during tcpdump shutdown (which is presumably why we
> haven't hit this more often on ASan/HWASan builds).
>
> i assume the fix is to disable the signal handlers before calling
> pcap_close() to free the struct pcap_t, but i thought i'd bring this
> up on the list before i (a) look at reproducing this locally and (b)
> send a patch...
>
> --- End Message ---
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