Unix Pageant: make stdout line-buffered in --debug mode.

My normal usage of --debug is to run it in a terminal, where it starts
by printing its SSH_AUTH_SOCK setting for me to paste into another
terminal to run test commands, and then follows that with diagnostic
logging of the requests it's receiving.

But if you'd rather get that diagnostic information in some location
other than a terminal – say, sent to a file which you're viewing in
'less' so that you can search back and forth in it, or piped to
another machine because your test requests are going to come from
somewhere out of sight of your monitor – then you might run 'pageant
--debug' with its stdout being a pipe or a file rather than a
terminal, in which case the standard stdio policy will make it
unbuffered, and the diagnostics won't show up in a timely manner.

The one-line code change is due to Marco Ricci, who had a rather
different motivation.

unix/pageant.c

@@ -1223,6 +1223,11 @@ void run_agent(FILE *logfp, const char *symlink_path)
     } else if (life == LIFE_PERM) {
         pageant_fork_and_print_env(false);
     } else if (life == LIFE_DEBUG) {
+        /* Force stdout to be line-buffered in preference to unbuffered, so
+         * that if diagnostic output is being piped somewhere, it will arrive
+         * promptly at the other end of the pipe */
+        setvbuf(stdout, NULL, _IOLBF, 0);
+
         pageant_print_env(getpid());
         upc->logfp = stdout;