weechat negotiates away-notify, but doesn't like the `MODE +a` lines

[slingamn]

@jesopo reports seeing weechat display lines like this:

18:32 -- User mode [+a] by irc.darwin.network
18:33 -- User mode [-a] by irc.darwin.network
18:40 -- User mode [+a] by irc.darwin.network
18:43 -- User mode [-a] by irc.darwin.network
19:06 -- User mode [-a] by irc.darwin.network

These are MODE lines sent by Oragono to clients that negotiate the away-notify cap, as Weechat does. Raw output:

@time=2019-04-28T18:06:53.190Z :irc.darwin.network MODE abrousse -a\r\n

Is this just a Weechat bug, or is our MODE line somehow malformed, or should we not be sending MODE lines (just the AWAY lines as described in the spec)?

[DanielOaks]

...hm dodgy. Yeah we should only be sending the AWAY lines and not the MODE ones.

[slingamn]

This was a regression from #458; we intended to duplicate the MODE line only to other attached sessions of the same client, but instead duplicated it to all friends.