bug#47478: 28.0.50; Provide customization option for nnimap-keepalive-timer

[Eric Abrahamsen]

Bob Newell requested the ability to customize the
`nnimap-keepalive-timer', and I've opened this bug report with a patch
that adds a new `nnimap-keepalive-times' customization option.

It lets you customize both function interval and period of inactivity,
or you can set it to nil to disable the keepalive altogether.

I also sneaked in a little extra change: since we provided the option to
use Dbus to close Gnus servers when your machine is going to sleep, I've
noticed that sometimes (more often than you'd think) I seem to be
sleeping the machine in between sending the keepalive NOOP and receiving
the response.

When the computer wakes, Emacs is hung while Gnus waits on an OK that
isn't coming, and I need to C-g to get out of it. I would have thought
this was a pretty small target to hit, particular since my imap servers
are all on this computer, but it happens quite often. So this patch also
lets `nnimap-streaming' to t around the keepalive command, so we don't
wait for the response. Semantically incorrect, but the effect is right.

If this is acceptable I can add a NEWS entry (and maybe the manual as
well?).

Eric

nnimap-keepalive-customization.diff
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