process-util: show requested process name in the log

This is useful for debugging issues like #26474.

src/basic/process-util.c

@@ -1185,7 +1185,7 @@ int safe_fork_full(
         else
                 pid = fork();
         if (pid < 0)
-                return log_full_errno(prio, errno, "Failed to fork: %m");
+                return log_full_errno(prio, errno, "Failed to fork off '%s': %m", strna(name));
         if (pid > 0) {
                 /* We are in the parent process */