sysoptions.h: Add ability to override DROPBEAR_LISTEN_BACKLOG

This change allows adding DROPBEAR_LISTEN_BACKLOG to localoptions.h to
force the value.

sysoptions.h

@@ -235,11 +235,13 @@
 #define DROPBEAR_VFORK 1
 #endif
+#ifndef DROPBEAR_LISTEN_BACKLOG
 #if MAX_UNAUTH_CLIENTS > MAX_CHANNELS
 #define DROPBEAR_LISTEN_BACKLOG MAX_UNAUTH_CLIENTS
 #else
 #define DROPBEAR_LISTEN_BACKLOG MAX_CHANNELS
 #endif
+#endif
 #ifndef DROPBEAR_NONE_CIPHER
 #define DROPBEAR_NONE_CIPHER 0