Add option to change shutdown/reboot keys (#487)

Include options in the configuration to change which function keys to use for shutdown and reboot.
Fix config.map_len size in src/config.c.
Add missing defaults in config_defaults() in src/config.c.

res/lang/en.ini

@@ -34,12 +34,12 @@ err_user_init = failed to initialize user
 err_user_uid = failed to set user UID
 err_xsessions_dir = failed to find sessions folder
 err_xsessions_open = failed to open sessions folder
-f1 = F1 shutdown
-f2 = F2 reboot
 login = login
 logout = logged out
 numlock = numlock
 password = password
+restart = reboot
 shell = shell
+shutdown = shutdown
 wayland = wayland
 xinitrc = xinitrc