Decrement SHLVL when running `exec`

`exec` removes fish from the shell "stack", so SHLVL needs to be
decremented to match. This means `exec fish` will result in the same
SHLVL in the new fish instance.

Also tweak the SHLVL logic to interpret an environment SHLVL of "3foo"
as garbage instead of as the value "3".

Fixes #1693.