Make sure SHLVL is working as expected

[pawamoy]

If it's not, we can still rely on the parents field to compute it.

It seems to be working fine at first glance:

$ # in bash
$ echo $SHLVL
1
$ zsh
$ echo $SHLVL
2

...though SHLVL is defined in the Bash man page as follow:

SHLVL Incremented by one each time an instance of bash is started.

Maybe it's also used the same way by Zsh.