LibC: stdlib: Add clearenv() function

Author: https://github.com/bcoles
Commit: 853664bd3c
Pull-request: https://github.com/SerenityOS/serenity/pull/4439

Base/usr/share/man/man3/clearenv.md

@@ -0,0 +1,34 @@
+## Name
+
+clearenv - clear the environment
+
+## Synopsis
+
+```**c++
+#include <stdlib.h>
+
+clearenv();
+```
+
+## Description
+
+Clears all environment variables and sets the external
+variable `environ` to NULL.
+
+## Return value
+
+The `clearenv()` function returns zero.
+
+## Examples
+
+```c++
+#include <stdlib.h>
+
+int main()
+{
+    clearenv();
+    putenv("PATH=/bin");
+
+    return 0;
+}
+```