Remove the implementation of setting keepalive settings on sockets and use
the implementation from sipyco instead.
Signed-off-by: Michael Birtwell <michael.birtwell@oxionics.com>
This partially reverts commit b5e1bd3fa2,
which had removed keepalive. This, however, led to experiments
hanging forever if the core device had dropped the connection
(e.g. to a kernel CPU panic, or the device being rebooted).
The chosen keepalive settings are fairly conservative (with the
10 s timeout) to avoid any possible interaction with smoltcp's
3 s ARP try interval (see GitHub issue #1150), even though this
should be a non-issue now due to the larger ARP cache.
This removes:
* host-side keepalive, which turns out not to be required
* custom connection timeout (the default is OK)
* SSH tunneling support (doesn't seem to be actually used anywhere)
