[posix] add details to UDP bind failure and log at Warn level (#13109)

This adds details to the Posix platform UDP bind error message, showing address and
port just like for the otPlatUdpConnect case. Also the severity is changed from Crit
to Warn, since it's not a critical failure given that otPlatUdpBind() is used in a
loop to find an available ephemeral port - i.e. probe the ports in range until one
succeeds.

It also fixes an issue where `errno` might be modified by the logging code itself.

Ideally the platform code would discern 'port in use' vs 'unrecoverable failure to
bind the port', but the currently defined OT APIs don't allow for any other errors
apart from ok/failed. If the specific port number is really needed, the caller
is responsible to log a critical failure.

src/posix/platform/udp.cpp

@@ -293,7 +293,10 @@ otError otPlatUdpBind(otUdpSocket *aUdpSocket)
 exit:
     if (error == OT_ERROR_FAILED)
     {
-        ot::Posix::Udp::LogCrit("Failed to bind UDP socket: %s", strerror(errno));
+        int err = errno;
+        ot::Posix::Udp::LogWarn("Failed to bind UDP socket to [%s]:%u: %s",
+                                Ip6AddressString(&aUdpSocket->mSockName.mAddress).AsCString(),
+                                aUdpSocket->mSockName.mPort, strerror(err));
     }
 
     return error;
@@ -420,12 +423,13 @@ otError otPlatUdpConnect(otUdpSocket *aUdpSocket)
 
     if (connect(fd, reinterpret_cast<struct sockaddr *>(&sin6), sizeof(sin6)) != 0)
     {
+        int err = errno;
 #ifdef __APPLE__
-        VerifyOrExit(errno == EAFNOSUPPORT && isDisconnect);
+        VerifyOrExit(err == EAFNOSUPPORT && isDisconnect);
 #endif
         ot::Posix::Udp::LogWarn("Failed to connect to [%s]:%u: %s",
                                 Ip6AddressString(&aUdpSocket->mPeerName.mAddress).AsCString(),
-                                aUdpSocket->mPeerName.mPort, strerror(errno));
+                                aUdpSocket->mPeerName.mPort, strerror(err));
         error = OT_ERROR_FAILED;
     }