nss-myhostname: expose the "outbound" IP addresses under the synthetic "_outbound" hostname

I found myself often looking for a quick way to determine "the local IP
address", and then being lost in the "ip addr" output to find for the
right one to use. This is supposed to help a bit with that. Let's
introduce a new special hostname "_outbound" with semantics similar to
"_gateway" that resolves to addresses that are the closest I could come
up with that maps to "the" local IP address.

src/basic/hostname-util.h

@@ -28,3 +28,8 @@ static inline bool is_gateway_hostname(const char *hostname) {
         /* This tries to identify the valid syntaxes for the our synthetic "gateway" host. */
         return STRCASE_IN_SET(hostname, "_gateway", "_gateway.");
 }
+
+static inline bool is_outbound_hostname(const char *hostname) {
+        /* This tries to identify the valid syntaxes for the our synthetic "outbound" host. */
+        return STRCASE_IN_SET(hostname, "_outbound", "_outbound.");
+}