[Server] Listen and serve on a unix socket

Allow to use a unix socket as a listener.
To specify an endpoint type we use an optional configuration
field 'net', as there's no way to distinguish a relative
socket path from a hostname.

Signed-off-by: Anton Tiurin <noxiouz@yandex.ru>

configuration/configuration_test.go

@@ -61,6 +61,7 @@ var configStruct = Configuration{
 	},
 	HTTP: struct {
 		Addr   string `yaml:"addr,omitempty"`
+		Net    string `yaml:"net,omitempty"`
 		Prefix string `yaml:"prefix,omitempty"`
 		Secret string `yaml:"secret,omitempty"`
 		TLS    struct {