Fix ggtags-navigation-mode-abort when pop-tag-mark errs
Fix #44: New variable ggtags-enable-navigation-keys

to control ggtags-navigation-map. Using ggtags-navigation-mode is
unsafe because it is used more than controlling ggtags-navigation-map.

Affect #40: ggtags-suppress-navigation-keys is removed; use
ggtags-enable-navigation-keys instead.