examples: fix compiling with MSVC

- `websocket.c`: use `Sleep()` on Windows.
  `sleep()` and `unistd.h` are not available in MSVC.

- `http2-upload.c`: use local `gettimeofday()` implementation when
  compiled with MSVC.
  (Alternate solution is to disable the trace function for MSVC.)
  Public domain code copied and adapted from libssh2:
  e973493f99/src/misc.c (L719-L743)

- silence compiler warning for deprecated `inet_addr()`.
  Also drop duplicate winsock2 include.
  ```
  curl\docs\examples\externalsocket.c(125,32): error C2220: the following warning is treated as an error [curl\bld\docs\examples\curl-example-externalsocket.vcxproj]
  curl\docs\examples\externalsocket.c(125,32): warning C4996: 'inet_addr': Use inet_pton() or InetPton() instead or define _WINSOCK_DEPRECATED_NO_WARNINGS to disable deprecated API warnings [curl\bld\docs\examples\curl-example-e
  ```
  Ref: https://github.com/curl/curl/actions/runs/9227337318/job/25389073450#step:4:95

- silence an MSVC compiler warning. This is in conflict with `checksrc`
  rules, so silence the rule in favour of the warning-free C syntax.
  ```
  curl\docs\examples\multi-legacy.c(152,1): error C2220: the following warning is treated as an error [curl\bld\docs\examples\curl-example-multi-legacy.vcxproj]
  curl\docs\examples\multi-legacy.c(152,1): warning C4706: assignment within conditional expression [curl\bld\docs\examples\curl-example-multi-legacy.vcxproj]
  ```
  Ref: https://github.com/curl/curl/actions/runs/9227337318/job/25389073450#step:4:226

- do not use `sys/time.h` and `unistd.h` in Windows builds.
  Some of these includes look unnecessary. Subject to another PR.

Cherry-picked from #13766
Closes #13771

docs/examples/multi-debugcallback.c

@@ -30,8 +30,10 @@
 #include <string.h>
 
 /* somewhat unix-specific */
+#ifndef _WIN32
 #include <sys/time.h>
 #include <unistd.h>
+#endif
 
 /* curl stuff */
 #include <curl/curl.h>