Merge bitcoin/bitcoin#28649: Do the SOCKS5 handshake reliably

af0fca530e netbase: use reliable send() during SOCKS5 handshake (Vasil Dimov)
1b19d1117c sock: change Sock::SendComplete() to take Span (Vasil Dimov)

Pull request description:

  The `Socks5()` function which does the SOCKS5 handshake with the SOCKS5 proxy sends bytes to the socket without retrying partial writes.

  `send(2)` may write only part of the provided data and return. In this case the caller is responsible for retrying the operation with the remaining data. Change `Socks5()` to do that. There is already a method `Sock::SendComplete()` which does exactly that, so use it in `Socks5()`.

  A minor complication for this PR is that `Sock::SendComplete()` takes `std::string` argument whereas `Socks5()` has `std::vector<uint8_t>`. Thus the necessity for the first commit. It is possible to do also in other ways - convert the data in `Socks5()` to `std::string` or have just one `Sock::SendComplete()` that takes `void*` and change the callers to pass `str.data(), str.size()` or `vec.data(), vec.size()`.

  This came up while testing https://github.com/bitcoin/bitcoin/pull/27375.

ACKs for top commit:
  achow101:
    ACK af0fca530e
  jonatack:
    ACK af0fca530e
  pinheadmz:
    ACK af0fca530e

Tree-SHA512: 1d4a53d0628f7607378038ac56dc3b8624ce9322b034c9547a0c3ce052eafb4b18213f258aa3b57bcb4d990a5e0548a37ec70af2bd55f6e8e6399936f1ce047a

src/net.cpp

@@ -3219,7 +3219,6 @@ bool CConnman::Start(CScheduler& scheduler, const Options& connOptions)
     // Start threads
     //
     assert(m_msgproc);
-    InterruptSocks5(false);
     interruptNet.reset();
     flagInterruptMsgProc = false;
 
@@ -3291,7 +3290,7 @@ void CConnman::Interrupt()
     condMsgProc.notify_all();
 
     interruptNet();
-    InterruptSocks5(true);
+    g_socks5_interrupt();
 
     if (semOutbound) {
         for (int i=0; i<m_max_outbound; i++) {