lib: simplify detection of when the user is leaving the CLI config mode

lib: simplify detection of when the user is leaving the CLI config mode

We can make use of the vty->config variable to know when the CLI
user is in the configuration mode or not. This is much simpler
than obtaining this information from the vty node, and also a more
robust solution (the three switch statements below, for example,
were out of sync).

Also, fix a bug where vty->config wasn't being unset in the
vty_config_exit() function (bug introduced by commit f344c66ea3).

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>