Closes #5849

Check readiness of all sockets before waiting on them
to avoid locking in case the one-time event FD_WRITE
was already consumed by a previous wait operation.

More information about WinSock network events:
https://docs.microsoft.com/en-us/windows/win32/api/
   winsock2/nf-winsock2-wsaeventselect#return-value

Closes #5634

This avoids using a pair of TCP ports to provide wakeup functionality
for every multi instance on Windows, where socketpair() is emulated
using a TCP socket on loopback which could in turn lead to socket
resource exhaustion.

A previous version of this patch failed to account for how in WinSock,
FD_WRITE is set only once when writing becomes possible and not again
until after a send has failed due to the buffer filling. This contrasts
to how FD_READ and FD_OOB continue to be set until the conditions they
refer to no longer apply. This meant that if a user wrote some data to
a socket, but not enough data to completely fill its send buffer, then
waited on that socket to become writable, we'd erroneously stall until
their configured timeout rather than returning immediately.

This version of the patch addresses that issue by checking each socket
we're waiting on to become writable with select() before the wait, and
zeroing the timeout if it's already writable.

Assisted-by: Marc Hörsken
Reviewed-by: Marcel Raad
Reviewed-by: Daniel Stenberg
Tested-by: Gergely Nagy
Tested-by: Rasmus Melchior Jacobsen
Tested-by: Tomas Berger

Replaces #5397
Reverts #5632
Closes #5634

Change Curl_socket_check to use select-fallback in Curl_poll
instead of implementing it in Curl_socket_check and Curl_poll.

Reviewed-by: Daniel Stenberg
Reviewed-by: Jay Satiro

Replaces #5262 and #5492
Closes #5707

This commit changes Curl_socket_check to use POLLPRI to
check for connect failure on the write socket, because
POLLPRI maps to fds_err. This is in line with select(2).

The select-based socket check correctly checks for connect
failures by adding the write socket also to fds_err.

The poll-based implementation (which internally can itself
fallback to select again) did not previously check for
connect failure by using POLLPRI with the write socket.

See the follow up commit to this for more information.

This commit makes sure connect failures can be detected
and handled if HAVE_POLL_FINE is defined, eg. on msys2-devel.

Reviewed-by: Daniel Stenberg
Reviewed-by: Jay Satiro

Replaces #5509
Prepares #5707

With Winsock the valid range is [0..INVALID_SOCKET-1] according to
https://docs.microsoft.com/en-us/windows/win32/winsock/socket-data-type-2

Reviewed-by: Jay Satiro
Reviewed-by: Marcel Raad
Reviewed-by: Daniel Stenberg

Closes #5760

Follow-up to 5620d2cc78c0

Works with --create-dirs and with -J

Add test 3008, 3009, 3011, 3012 and 3013 to verify.

Closes #5637