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/test/fuzz/util.h

@@ -47,6 +47,13 @@ class FuzzedSock : public Sock
      */
     mutable std::optional<uint8_t> m_peek_data;
 
+    /**
+     * Whether to pretend that the socket is select(2)-able. This is randomly set in the
+     * constructor. It should remain constant so that repeated calls to `IsSelectable()`
+     * return the same value.
+     */
+    const bool m_selectable;
+
 public:
     explicit FuzzedSock(FuzzedDataProvider& fuzzed_data_provider);
 
@@ -72,6 +79,10 @@ public:
 
     int GetSockName(sockaddr* name, socklen_t* name_len) const override;
 
+    bool SetNonBlocking() const override;
+
+    bool IsSelectable() const override;
+
     bool Wait(std::chrono::milliseconds timeout, Event requested, Event* occurred = nullptr) const override;
 
     bool WaitMany(std::chrono::milliseconds timeout, EventsPerSock& events_per_sock) const override;