fixed a logic bug in the tevent nesting code

fixed a logic bug in the tevent nesting code

The event nesting code never triggered as nesting.level was never
greater than 1. The main event loop needs to increase the nesting
level by 1.

I also added a paranoia check to the nesting setup call. The API as
currently written cannot support multiple nesting hooks, so we need to
abort if multiple hooks are tried.