libmilter: 8.15.2 -> 8.17.1
Changelog:
```
[...]
8.16.1/8.16.1 2020/07/05
[...]
LIBMILTER: Fix typo in a macro. Patch from Ignacio Goyret
of Alcatel-Lucent.
LIBMILTER: Fix reference in xxfi_negotiate documentation.
Patch from Sven Neuhaus.
LIBMILTER: Fix function name in smfi_addrcpt_par documentation.
Patch from G.W. Haywood.
LIBMILTER: Fix a potential memory leak in smfi_setsymlist().
Patch from Martin Svec.
[...]
```