Add reasonable default device names for divecomputer import

So far we hard coded /dev/ttyUSB0 - which is a good starting point in
Linux but not so useful on Windows or MacOS. This was now moved into one
of our OS helper functions with (somewhat) reasonable defaults.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

display-gtk.h

@@ -32,6 +32,8 @@ extern void subsurface_set_conf(char *name, pref_type_t type, const void *value)
 extern const void *subsurface_get_conf(char *name, pref_type_t type);
 extern void subsurface_close_conf(void);
 
+extern const char *subsurface_USB_name(void);
+
 extern visible_cols_t visible_cols;
 
 extern const char *divelist_font;

gtk-gui.c

@@ -913,7 +913,7 @@ static GtkEntry *dive_computer_device(GtkWidget *vbox)
 
 	entry = gtk_entry_new();
 	gtk_container_add(GTK_CONTAINER(frame), entry);
-	gtk_entry_set_text(GTK_ENTRY(entry), "/dev/ttyUSB0");
+	gtk_entry_set_text(GTK_ENTRY(entry), subsurface_USB_name());
 
 	return GTK_ENTRY(entry);
 }

linux.c

@@ -46,3 +46,8 @@ void subsurface_close_conf(void)
 {
 	/* this is a no-op */
 }
+
+const char *subsurface_USB_name()
+{
+	return("/dev/ttyUSB0");
+}

macos.c

@@ -89,3 +89,8 @@ void subsurface_close_conf(void)
 	CFRelease(xmlData);
 	CFRelease(propertyList);
 }
+
+const char *subsurface_USB_name()
+{
+	return("/dev/tty.SLAB_USBtoUART");
+}

windows.c

@@ -78,3 +78,8 @@ void subsurface_close_conf(void)
 		printf("RegFlushKey failed \n");
 	RegCloseKey(hkey);
 }
+
+const char *subsurface_USB_name()
+{
+	return("COM3");
+}