socket: introduce SOCK_EAGAIN() and use it

To contain the logic of checking for both `EWOULDBLOCK` and/or `EAGAIN`
depending on platform/availability. Also to avoid checking for both if
they mapp to the same value, and to avoid PP guards around use.

This also ensures `EAGAIN` is consistently not checked on Windows, where
headers defined it, but `SOCKERRNO` never returns it, because curl maps
it to `WSAGetLastError()`.

If they map to the same value, checking them both in an `if` expression
trips GCC warning `-Wlogical-op` (the same way it triggers duplicate
case value error in `switch`).

Also:
- replace two `switch()` statements with the new macro.
- tests/server/sws: make two outliers use the new macro that were only
  checking for `EWOULDBLOCK` before this patch, in `connect_to()`.
- move variables to the left-side of expressions, where missing.
- rustls: use a variant of this macro that uses raw `EWOULDBLOCK`.
  Tried tracing it back to the origins, but I couldn't figure out if
  this is working as expected on all supported Windows versions in
  Rust. It seems to be using `GetLastError()`, according to
  https://docs.rs/system_error/0.2.0/system_error/, which would be
  probably incorrect.

Notes:
- it's probably a good idea to assign `SOCKERRNO` to a variable before
  passing it to this macro.

Cherry-picked from #21893

Closes #21992

src/tool_cb_rea.c

@@ -109,7 +109,7 @@ size_t tool_read_cb(char *buffer, size_t sz, size_t nmemb, void *userdata)
 #ifndef CURL_WINDOWS_UWP
     rc = sread(per->infd, buffer, curlx_uztosi(sz * nmemb));
     if(rc < 0) {
-      if(SOCKERRNO == SOCKEWOULDBLOCK) {
+      if(SOCK_EAGAIN(SOCKERRNO)) {
         errno = 0;
         config->readbusy = TRUE;
         return CURL_READFUNC_PAUSE;