DnsProxyListener: Support pre-4.3 message formats

Android 4.3 added a new "iface" argument to the getaddrinfo() and
gethostbyaddr() calls.  Support both 4.2 and 4.3 message formats in the
proxy.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
dnsproxy2: Fix socket permissions and error handling

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
dnsproxy2: Add "-u" options to force a specific fsuid for all DNS requests

This is mainly useful for testing purposes.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>