hostnamectl: show hint when user try to set transient hostname but static hostname is already used

src/shared/hostname-setup.c

@@ -233,4 +233,4 @@ static const char* const hostname_source_table[] = {
         [HOSTNAME_FALLBACK]  = "fallback",
 };
 
-DEFINE_STRING_TABLE_LOOKUP_TO_STRING(hostname_source, HostnameSource);
+DEFINE_STRING_TABLE_LOOKUP(hostname_source, HostnameSource);

src/shared/hostname-setup.h

@@ -11,7 +11,9 @@ typedef enum HostnameSource {
         _HOSTNAME_INVALID = -1,
 } HostnameSource;
 
-const char* hostname_source_to_string(HostnameSource source);
+const char* hostname_source_to_string(HostnameSource source) _const_;
+HostnameSource hostname_source_from_string(const char *str) _pure_;
+
 int sethostname_idempotent(const char *s);
 
 int shorten_overlong(const char *s, char **ret);