broker: avoid truncation of log messages during shutdown

Problem: In the broker shutdown code, log messages may be
truncated because the log forwarding code uses a static buffer of
FLUX_MAX_LOGBUF.

Since a maximum size of the final log message is known, use a
variable length array for the buffer in forward_logbuf() instead
of FLUX_MAX_LOGBUF to avoid any truncation.

src/broker/shutdown.c

@@ -66,19 +66,14 @@ static int forward_logbuf (flux_t *h,
     struct stdlog_header hdr;
     const char *txt;
     size_t txtlen;
-    char buf[FLUX_MAX_LOGBUF];
+    int len = strlen (stdlog);
+    char buf[len + 1];
     int loglevel;
 
     if (flux_msg_unpack (request, "{s:i}", "loglevel", &loglevel) < 0)
         loglevel = LOG_ERR;
 
-    if (stdlog_decode (stdlog,
-                       strlen (stdlog),
-                       &hdr,
-                       NULL,
-                       NULL,
-                       &txt,
-                       &txtlen) < 0
+    if (stdlog_decode (stdlog, len, &hdr, NULL, NULL, &txt, &txtlen) < 0
         || STDLOG_SEVERITY (hdr.pri) > loglevel
         || snprintf (buf,
                      sizeof (buf),