function: restore '%self' functionality for --on-process-exit

[zanchey]

This PR restores "%self" as an argument for --on-process/--on-process-exit.

It also adds "self" as a more sensible alias, but perhaps this could be dropped in favour of using $fish_pid.

[zanchey]

%self gets a reasonable amount of use and this saves a few of them from exploding - notably virtualfish.

[mqudsi]

Thanks for doing this, @zanchey. Isn't --on-process-exit on its way out, though? I'd be fine with restoring compatibility for %self for now, but given that it's being deprecated in favor of events, I wouldn't expand its functionality any more than that.

Perhaps we should also print a warning message about the deprecation of %self and --on-process-exit to the tty...

[ridiculousfish]

What's the alternative way of doing this with events?

[faho]

@ridiculousfish: The alternative to function foo --on-process-exit %self is function foo --on-event fish_exit.

I added that one in b67a614 (which does not actually "replace" anything, don't believe the commit message).

[faho]

@zanchey: I think we don't want to add additional functionality to this, since I want to get away from it - and I haven't heard any disagreement.

So I'd merge it without the self part, unless @ridiculousfish wants to add the % stuff to #4940.

[zanchey]

SGTM.

[faho]

Merged without the "self" stuff as 21890cc.