Rename internal functions for consistency

The function fish_command_not_found_setup shows up in the completion list when trying to complete fish\<tab\>. Since this function is never accessed by the user, and erased once executed, it's probably reasonable to rename it "__fish_command_not_found_setup" as the other internal functions.
The same thing applies to fish_command_not_found_handler, which is defined inside the previous function (and shows up in the completion list in place of it).
I also fixed some typos I randomly found.