Add new defvoo nnimap-keepalive-intervals to Gnus nnimap servers

* lisp/gnus/nnimap.el (nnimap-keepalive-intervals): New per-server
config for customizing when keepalive commands are sent.
(nnimap-keepalive, nnimap-open-connection-1): Consult in these
places.  Additionally, use nnimap-streaming -> t when sending the
keepalive NOOP, so we don't wait for the response.
* doc/misc/gnus.texi (Customizing the IMAP Connection): Document.
10 jobs for master in 136 minutes and 26 seconds (queued for 2 seconds)
Status Job ID Name Coverage
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passed #18908
prep-image-base

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build-image-inotify

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build-image-filenotify-gio

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passed #18911
build-image-gnustep

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  Platforms
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test-filenotify-gio

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passed #18913
test-gnustep

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build-native-bootstrap-speed0
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build-native-bootstrap-speed1
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build-native-bootstrap-speed2
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test-all-inotify
 
Name Stage Failure
failed
test-filenotify-gio Platforms

make: Leaving directory '/checkout/test'
Running after_script
Running after script...
$ test -n "$(docker ps -aq -f name=${test_name})" && docker cp ${test_name}:checkout/test ${test_name}
$ test -n "$(docker ps -aq -f name=${test_name})" && docker rm ${test_name}
test-filenotify-gio-aeada12e
Cleaning up file based variables
ERROR: Job failed: exit code 2