core: don't reset /dev/console if stdin/stdout/stderr as passed as fd in a transient service

Otherwise we might end resetting /dev/console all the time when a transient service starts or stops.

Fixes #2377
Fixes #2198
Fixes #2061

src/core/execute.h

@@ -122,6 +122,8 @@ struct ExecContext {
 
         nsec_t timer_slack_nsec;
 
+        bool stdio_as_fds;
+
         char *tty_path;
 
         bool tty_reset;