userland: move posix/environ from cpp-cheat

userland/posix/environ.c

@@ -0,0 +1,15 @@
+/* Print all environment variables. */
+
+#include <stdio.h>
+#include <stdlib.h>
+
+extern char **environ;
+
+int main(void) {
+    char **env = environ;
+    while (*env) {
+        printf("%s\n", *env);
+        env++;
+    }
+    return EXIT_SUCCESS;
+}