Fix advanceKeyboardFocus in cZ

I've spotted that the function advanceKeyboardFocus is comparing against a non-existant variable (userList) so this should be fixed.
Whilst here tidy up some other code that looks up the element "user-list" which is already stored in client.list so we should just use that instead.

[Approval Request Comment]
Regression caused by (bug #): n/a
User impact if declined: advancing focus via F6 doesn't work properly
Testing completed (on m-c, etc.): 2.53.22pre
Risk to taking this patch (and alternatives if risky): low
String changes made by this patch: none

Comment on attachment 9493473 [details] [diff] [review]
1971036-irc-fix-userList-25322.patch

LGTM r/a+

Unrelated: Seems only isMac is used in handlers.js. Room for a later cleanup.

Pushed by frgrahl@gmx.net:
https://hg.mozilla.org/comm-central/rev/f473a6263690
Fix advanceKeyboardFocus in cZ. r=frg DONTBUILD

https://gitlab.com/seamonkey-project/seamonkey-2.53-comm/-/commit/6ef02b4a413e03f45afc47e79798687ede09a3ba
Fix advanceKeyboardFocus in cZ. r=frg a=frg