Separate on-job-exit and and on-process-exit events

It is possible to run a function when a process exits via `function
--on-process-exit`, or when a job exits via `function --on-job-exits`.
Internally these were distinguished by the pid in the event: if it was
positive, then it was a process exit. If negative, it represents a pgid
and is a job exit. If zero, it fires for both jobs and processes, which is
pretty weird.

Switch to tracking these explicitly. Separate out the --on-process-exit
and --on-job-exit event types into separate types. Stop negating pgids as
well.

tests/checks/signal.fish

@@ -28,6 +28,11 @@ function anychild --on-process-exit 0
     echo $argv[1] $argv[3]
 end
 
+function anyjob --on-job-exit 0
+    # Type and exit status
+    echo $argv[1] $argv[3]
+end
+
 echo "command false:"
 command false
 # CHECK: command false: