Update ahost man page to describe -s option.

Commit ee22246507c9 added the -s <domain> option to the
ahost command, but neglected to update the man page to
describe it.

Also fix typo in description of -t option.
Move memset call below platform-specific declarations

A GitHub commenter [1] says that my recent change to ahost.c has
problems compiling on Windows + C89 platforms.

[1]  https://github.com/bagder/c-ares/commit/ee22246507c9#commitcomment-6587616
host_callback: Fall back to AF_INET on searching with AF_UNSPEC

Previously, when an ares_gethostbyname() searched with AF_UNSPEC and the
first AF_INET6 call only returned CNAMEs, the host_callback never
retried AF_INET.

This patch makes sure than on ARES_SUCCESS, the result of AF_INET6 is
taken as authoritative only if the result contains some addresses.
Add -t u option to ahost

Add an option to allow specification of the AF_UNSPEC
address family.
CONTRIBUTING: add file to indicate mailing list is preferred
Comment in ares_ipv6.h