cmake: fix ENABLE_WERROR=ON breaking auto-detections

- cmake: fix compiler warnings in `CheckNonblockingSocketSupport`.
  detection functions.

  Without this, these detections fail when `ENABLE_WERROR=ON`.

- cmake: disable ENABLE_WERROR for MSVC during symbol checks in `src`.

  CMake's built-in symbol check function `check_symbol_exists()`
  generate warnings with MSVC. With warnings considered errors, these
  detections fail permanently. Our workaround is to disable
  warnings-as-errors while running these checks.

  ```
  CheckSymbolExists.c(8): warning C4054: 'type cast': from function pointer '__int64 (__cdecl *)(const char *,char **,int)' to data pointer 'int *'
  in `return ((int*)(&strtoll))[argc];`
  ```

  Ref: https://ci.appveyor.com/project/libssh2org/libssh2/builds/46537222/job/4vg4yg333mu2lg9b

- example: replace `strcasecmp()` with C89 `strcmp()`.

  To avoid using CMake symbol checks in `example`.

  Another option is to duplicate the `check_symbol_exists()` workaround
  from `src`, but I figure it's not worth the complexity. We use
  `strcasecmp()` solely to check optional command-line options for
  example programs, and those are fine as lower-case.

  Without this, these detections fail when `ENABLE_WERROR=ON`.

- also delete `__function__` detection/use in `example`.

  To avoid the complexity for the sake of using it at a single place in
  of the example's error branch. Replace that use with a literal name of
  the function.

- cmake: also use `CMakePushCheckState` functions instead of manual
  save/restore.

Closes #857

example/sftp.c

@@ -209,13 +209,13 @@ int main(int argc, char *argv[])
 
     /* if we got an 4. argument we set this option if supported */
     if(argc > 5) {
-        if((auth_pw & 1) && !strcasecmp(argv[5], "-p")) {
+        if((auth_pw & 1) && !strcmp(argv[5], "-p")) {
             auth_pw = 1;
         }
-        if((auth_pw & 2) && !strcasecmp(argv[5], "-i")) {
+        if((auth_pw & 2) && !strcmp(argv[5], "-i")) {
             auth_pw = 2;
         }
-        if((auth_pw & 4) && !strcasecmp(argv[5], "-k")) {
+        if((auth_pw & 4) && !strcmp(argv[5], "-k")) {
             auth_pw = 4;
         }
     }