System test for PROXYv2

This commit adds a system test suite for PROXYv2. The idea on which it
is based is simple:

1. Firstly we check that 'allow-proxy' and 'allow-proxy-on' (whatever
is using the new 'isc_nmhandle_real_localaddr/peeraddr()') do what
they intended to do.

2. Anything else that needs an interface or peer address (ACL
functionality, for example) is using the old
'isc_nmhandle_localaddr/peeraddr()' - which are now returning
addresses received via PROXY (if any) instead of the real connection
addresses. The beauty of it that we DO NOT need to verify every bit of
the code relying on these functions: whatever works in one place will
work everywhere else, as these were the only functions that allowed
any higher level code to get peer and interface addresses.

This way it is relatively easy to see if PROXYv2 works as intended.

bin/tests/system/proxy/self-signed-key.pem

@@ -0,0 +1,8 @@
+-----BEGIN EC PARAMETERS-----
+BggqhkjOPQMBBw==
+-----END EC PARAMETERS-----
+-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEIHwX7CofJJGLEG6evl6h9WSmZ1q1HAOO+SKgvMiqCFcXoAoGCCqGSM49
+AwEHoUQDQgAE1axzFeiE1MGNxbiuEoNlsuLsiojtZh7mU5LUKu6R9gm4wzX6qNif
+M7paDmpPCKf8K+vj8ZOb3kjridks9DMLFg==
+-----END EC PRIVATE KEY-----