qga/commands-posix: Use getifaddrs when available

Currently, commands-posix.c assumes that getifaddrs() is only
available on Linux, and so the related guest agent command
guest-network-get-interfaces is only implemented for #ifdef __linux__.
This function does exist on other platforms, though, such as Solaris.
So, add a meson check for getifaddrs(), and move the code for
guest-network-get-interfaces to be built whenever getifaddrs() is
available.

The implementation for guest-network-get-interfaces still has some
Linux-specific code, which is not fixed in this commit. This commit
moves the relevant big chunks of code around without changing them, so
a future commit can change the code in place.

Signed-off-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Message-Id: <20220426195526.7699-2-adeason@sinenomine.net>