Prior to this change, a function with an on-job-exit event handler must be
added with the pgid of the job. But sometimes the pgid of the job is fish
itself (if job control is disabled) and the previous commit made last_pid
an actual pid from the job, instead of its pgroup.
Switch on-job-exit to accept any pid from the job (except fish itself).
This allows it to be used directly with $last_pid, except that it now
works if job control is off. This is implemented by "resolving" the pid to
the internal job id at the point the event handler is added.
Also switch to passing the last pid of the job, rather than its pgroup.
This aligns better with $last_pid.
Previously, when a command wasn't found, fish would emit the
"fish_command_not_found" *event*.
This was annoying as it was hard to override (the code ended up
checking for a function called `__fish_command_not_found_handler`
anyway!), the setup was ugly,
and it's useless - there is no use case for multiple command-not-found handlers.
Instead, let's just call a function `fish_command_not_found` if it
exists, or print the default message otherwise.
The event is completely removed, but because a missing event is not an error
(MEISNAE in C++-speak) this isn't an issue.
Note that, for backwards-compatibility, we still keep the default
handler function around even tho the new one is hard-coded in C++.
Also, if we detect a previous handler, the new handler just calls it.
This way, the backwards-compatible way to install a custom handler is:
```fish
function __fish_command_not_found_handler --on-event fish_command_not_found
# do a little dance, make a little love, get down tonight
end
```
and the new hotness is
```fish
function fish_command_not_found
# do the thing
end
```
Fixes#7293.
