Problem: ipc connect can fail on Windows, even after bind

[minrk]

Solution:

• move tcpip fallback to after connect instead of assuming successful ipc bind means ipc works (it's very weird and possibly a Windows bug that this can happen, but I can at least confirm that it does in very specific scenarios - Python from Windows Store + non-ascii username + default $env:TMP)
• correct some Windows error code checks
• set errno when connect fails (missing errno resulted in treating failure as success, leading to Problem: epoll crashes for some Windows users #4730)
• make sure to clear _w back to retired_fd after closing it

Also addresses todo item to remember the tcpip fallback after ipc has failed once, so the failing path doesn't get retried every time. I can back that out if folks want.

I believe this actually closes #4730, though after some debugging, I do believe there are still missing checks to signaler::valid(), because even when make_fd_pair properly fails, send is still called on the invalid signaler, leading to the EBADFD reported in #4730, when it seems like it should probably be an assert in the signaler constructor (or immediately after calling the constructor) that it failed to create an fd pair.

[bluca]

Formatting with clang-format (using clang-format) and showing differences with latest commit
diff --git a/src/ip.cpp b/src/ip.cpp
index db10b13..30d70eb 100644
--- a/src/ip.cpp
+++ b/src/ip.cpp
@@ -24,8 +24,9 @@
 #ifdef ZMQ_HAVE_IPC
 #include "ipc_address.hpp"
 // Don't try ipc if it fails once
-namespace zmq {
-    static bool try_ipc_first = true;
+namespace zmq
+{
+static bool try_ipc_first = true;
 }
 #endif

[minrk]

The truly bizarre property of this situation is that IPC totally does work, but it doesn't work in the default temp directory for some process configurations and usernames. In the situations where I've reproduced this, setting $env:TMP to a location outside $HOME/AppData, e.g. $HOME/myAppData fixes everything, but any subdir of $HOME/AppData does not. I'm not sure if there's a property that can be interrogated to pick a different directory, or if just this catch on connect is enough. It's really weird that bind succeeds and connect fails.

[BtbN]

This introduced a compilation error:

15.95 /50-libzmq/src/ip.cpp: In function 'int zmq::make_fdpair(fd_t*, fd_t*)':
15.95 /50-libzmq/src/ip.cpp:678:1: error: jump to label 'try_tcpip'
15.95   678 | try_tcpip:
15.95       | ^~~~~~~~~
15.95 /50-libzmq/src/ip.cpp:568:14: note:   from here
15.95   568 |         goto try_tcpip;
15.95       |              ^~~~~~~~~
15.95 /50-libzmq/src/ip.cpp:572:18: note:   crosses initialization of 'const SOCKET listener'
15.95   572 |     const SOCKET listener = open_socket (AF_UNIX, SOCK_STREAM, 0);
15.95       |                  ^~~~~~~~