net: isolate the protocol-agnostic part of CConnman::AcceptConnection()

Summary:
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

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11015