Isolate the second half of CConnman::AcceptConnection() into a new
separate method, which could be reused if we accept incoming connections
by other means than accept() (first half of
CConnman::AcceptConnection()).
This is a backport of core#20685 [6/20]
https://github.com/bitcoin/bitcoin/pull/20685/commits/7c224fdac4699a2c4953b33ab423f9cddbf95cf7
Depends on D11014