Fix MSVC level 1 (severe) warnings

We fix (hopefully) all instances of:

* <https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4005>
* <https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4024>
* <https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4028>
* <https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4047>
* <https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4087>
* <https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4090>
* <https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4273>
* <https://docs.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-1-c4312>

`zend_llist_add_element()` and `zend_llist_prepend_element()` now
explicitly expect a *const* pointer.

We use the macro `ZEND_VOIDP()` instead of a `(void*)` cast to suppress
C4090; this should prevent accidential removal of the cast by
clarifying the intention, and makes it easier to remove the casts if
the issue[1] will be resolved sometime.

[1] <https://developercommunity.visualstudio.com/content/problem/390711/c-compiler-incorrect-propagation-of-const-qualifie.html>

sapi/apache2handler/php_functions.c

@@ -17,6 +17,12 @@
 #define ZEND_INCLUDE_FULL_WINDOWS_HEADERS
 
 #include "php.h"
+#ifdef strcasecmp
+# undef strcasecmp
+#endif
+#ifdef strncasecmp
+# undef strncasecmp
+#endif
 #include "zend_smart_str.h"
 #include "ext/standard/info.h"
 #include "ext/standard/head.h"