Make BtIO default security level MEDIUM

MEDIUM makes more sense than the kernel default LOW which maps to "no
bonding" with SSP (something that's useful only for very special cases
such as OPP).
diff --git a/src/btio.c b/src/btio.c
index 88fa075..904c4ac 100644
--- a/src/btio.c
+++ b/src/btio.c
@@ -628,6 +628,7 @@
 	/* Set defaults */
 	opts->defer = DEFAULT_DEFER_TIMEOUT;
 	opts->master = -1;
+	opts->sec_level = BT_IO_SEC_MEDIUM;
 
 	while (opt != BT_IO_OPT_INVALID) {
 		switch (opt) {