res_smdr_whozz: Decline to load if 'device' setting not set.

This is a rather niche module that almost nobody will use;
therefore, comment this setting in the sample config and
decline to load if it isn't specified. This way, even with
the default sample configs installed, the module won't load
unless the config has been modified by the user. This
avoids a bunch of endless warnings in the console about
failing to write to the serial port when it likely hasn't
been configured for this use case.

Author: InterLinked1

configs/samples/res_smdr_whozz.conf.sample

@@ -10,6 +10,7 @@
 
 ;[general]
 ;device=/dev/ttyS0 ; Device name of serial port to which WHOZZ Calling? unit is connected (9600/N/1)
+                   ; This setting has no default and must be explicitly specified to use this module.
 
 ;[line-1]
 ;line = 1 ; Line number as configured on WHOZZ Calling? unit.

res/res_smdr_whozz.c

@@ -48,12 +48,14 @@
 		<synopsis>WHOZZ Calling? SMDR</synopsis>
 		<configFile name="res_smdr_whozz.conf">
 			<configObject name="general">
-				<configOption name="device" default="/dev/ttyS0">
+				<configOption name="device">
 					<synopsis>Serial device</synopsis>
 					<description>
 						<para>Serial device to use.</para>
 						<para>Currently, only one serial device is supported. The units themselves allow them to be chained together
 						so that only one serial connection to a PC is required.</para>
+						<para>This setting has no default and must be explicitly specified.
+						The default serial port on most systems is <literal>/dev/ttyS0</literal>.</para>
 					</description>
 				</configOption>
 			</configObject>
@@ -149,7 +151,7 @@ static AST_RWLIST_HEAD_STATIC(lines, whozz_line);
 static pthread_t serial_thread = AST_PTHREADT_NULL;
 static int thread_running = 0;
 
-static char serial_device[256] = "/dev/ttyS0"; /* Default serial port on a Linux system (especially if there's only one) */
+static char serial_device[256] = ""; /* No default setting, but default serial port on a Linux system is /dev/ttyS0 (especially if there's only one) */
 static int serial_fd = -1;
 static int unloading = 0;
 
@@ -1177,6 +1179,12 @@ static int load_module(void)
 		return AST_MODULE_LOAD_DECLINE;
 	}
 
+	/* If no serial device explicitly specified, decline to load. */
+	if (ast_strlen_zero(serial_device)) {
+		unload_module();
+		return AST_MODULE_LOAD_DECLINE;
+	}
+
 	/* 9600/N/1 */
 	serial_fd = open(serial_device, O_RDWR | O_NONBLOCK | O_NOCTTY);
 	if (serial_fd < 0) {