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 [5/20]
https://github.com/bitcoin/bitcoin/pull/20685/commits/7c224fdac4699a2c4953b33ab423f9cddbf95cf7
Depends on D11014