hostname-util: add new helper split_user_at_host()

Let's introduce a common helper for splitting user@host specifications
like we use them for --machine=.

src/basic/hostname-util.h

@@ -38,3 +38,5 @@ static inline bool is_dns_proxy_stub_hostname(const char *hostname) {
 }
 
 int get_pretty_hostname(char **ret);
+
+int split_user_at_host(const char *s, char **ret_user, char **ret_host);