MINOR: sock: rename sock_accept_conn() to sock_accepting_conn()

MINOR: sock: rename sock_accept_conn() to sock_accepting_conn()

This call was introduced by commit 5ced3e887 ("MINOR: sock: add
sock_accept_conn() to test a listening socket") but is actually quite
confusing because it makes one think the socket will accept a connection
(which is what we want to have in a new function) while it only tells
whether it's configured to accept connections. Let's call it
sock_accepting_conn() instead.

The same change was applied to sockpair which had the same issue.