Drop compat "gateway" name

Back in 5248e7e1f1 (July 2017) we moved over to
"_gateway", with the old name declared to be temporary measure. Since we're
doing a bunch of changes to resolved now, it seems to be a good moment to make
this simplification and not add support for the compat name in new code.

src/basic/hostname-util.c

@@ -212,20 +212,6 @@ bool is_localhost(const char *hostname) {
                 endswith_no_case(hostname, ".localhost.localdomain.");
 }
 
-bool is_gateway_hostname(const char *hostname) {
-        assert(hostname);
-
-        /* This tries to identify the valid syntaxes for the our
-         * synthetic "gateway" host. */
-
-        return
-                strcaseeq(hostname, "_gateway") || strcaseeq(hostname, "_gateway.")
-#if ENABLE_COMPAT_GATEWAY_HOSTNAME
-                || strcaseeq(hostname, "gateway") || strcaseeq(hostname, "gateway.")
-#endif
-                ;
-}
-
 int sethostname_idempotent(const char *s) {
         char buf[HOST_NAME_MAX + 1] = {};
 

src/basic/hostname-util.h

@@ -5,6 +5,7 @@
 #include <stdio.h>
 
 #include "macro.h"
+#include "strv.h"
 
 bool hostname_is_set(void);
 
@@ -19,7 +20,11 @@ char* hostname_cleanup(char *s);
 #define machine_name_is_valid(s) hostname_is_valid(s, false)
 
 bool is_localhost(const char *hostname);
-bool is_gateway_hostname(const char *hostname);
+
+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.");
+}
 
 int sethostname_idempotent(const char *s);