sendqueue_length no longer determines if we check for socket-can-write, but sendpacket() sets a timestamp if it happened

network.*:
- sendpacket(): checks result and on EAGAIN, stores time of this event
- networking_wait_prepare(): uses the time if set to check up to 500ms later for the socket to be writeable again
- networking_wait_cleanup(): clears the time if the socket was writeable
- fixed some accidental tabs

Messenger.*, tox.*:
- pass-through functions to reach networking_wait_cleanup()
- fixed some accidental tabs