execute: let's close glibc syslog channels too

Just in case something opened them, let's make sure glibc invalidates
them too.

Thankfully so far no library opened log channels behind our back, at
least as far as I know, hence this is actually a NOP, but let's better
be safe than sorry.

src/core/execute.c

@@ -2464,6 +2464,9 @@ static int exec_child(
         log_forget_fds();
         log_set_open_when_needed(true);
 
+        /* In case anything used libc syslog(), close this here, too */
+        closelog();
+
         n_fds = n_storage_fds + n_socket_fds;
         r = close_remaining_fds(params, runtime, dcreds, user_lookup_fd, socket_fd, fds, n_fds);
         if (r < 0) {