Merge bitcoin/bitcoin#25421: net: convert standalone IsSelectableSocket() and SetSocketNonBlocking() to Sock methods

b527b54950 net: convert standalone SetSocketNonBlocking() to Sock::SetNonBlocking() (Vasil Dimov)
29f66f7682 moveonly: move SetSocketNonBlocking() from netbase to util/sock (Vasil Dimov)
b4bac55679 net: convert standalone IsSelectableSocket() to Sock::IsSelectable() (Vasil Dimov)
5db7d2ca0a moveonly: move IsSelectableSocket() from compat.h to sock.{h,cpp} (Vasil Dimov)

Pull request description:

  _This is a piece of #21878, chopped off to ease review._

  * convert standalone `IsSelectableSocket()` to `Sock::IsSelectable()`
  * convert standalone `SetSocketNonBlocking()` to `Sock::SetNonBlocking()`

  This further encapsulates syscalls inside the `Sock` class and makes the callers mockable.

ACKs for top commit:
  jonatack:
    ACK b527b54950 review/debug build/unit tests at each commit, cross-referenced the changes with `man select` and `man errno`, ran a signet node on the last commit with ip4/ip6//tor/i2p/cjdns and network connections were nominal
  dergoegge:
    Code review ACK b527b54950

Tree-SHA512: af783ce558c7a89e173f7ab323fb3517103d765c19b5d14de29f64706b4e1fea3653492e8ea73ae972699986aaddf2ae72c7cfaa7dad7614254283083b7d2632

src/net.cpp

@@ -970,8 +970,7 @@ void CConnman::CreateNodeFromAcceptedSocket(std::unique_ptr<Sock>&& sock,
         return;
     }
 
-    if (!IsSelectableSocket(sock->Get()))
-    {
+    if (!sock->IsSelectable()) {
         LogPrintf("connection from %s dropped: non-selectable socket\n", addr.ToString());
         return;
     }