Some Linux OSs impose a length limit on the path a Unix socket may have. This
is not an issue in `lightningd` since we `chdir()` into that directory before
opening the socket, however in pyln this became a problem for some tests,
since we use absolute paths in the testing framework. It's also a rather
strange quirk to expose to users.
This patch introduces a `UnixSocket` abstraction that attempts to work around
these limitations by aliasing the directory containing the socket into
`/proc/self/fd` and then connecting using that alias.
It was inspired by Open vSwitch code here https://github.com/openvswitch/ovs/blob/master/python/ovs/socket_util.py
Signed-off-by: Christian Decker <@cdecker>