resolve stricter warnings

In order to convert this repository to Meson, we need to make it
compile under `warning_level=3`.  Fix a number of warning classes
across the repository or disable them.

Some fixes are:

* Add missing header files.
* Fully initialize structs as necessary.
* Add `__attribute__((unused))` on parameters as necessary.
* Fix comparisons between signed and unsigned.
* Fix printf specifiers as necessary.
* Avoid case-fallthrough.
* Remove if conditions which are always true.

Some warnings would require extensive code changes, due to their
pervasive use, and so are disabled at a per-file level:
* `-Wpointer-arith`
* `-Wunused-result`

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: If8992b9108f12b39f796ed090ba29868c9f3c627
diff --git a/vpnor/backend.cpp b/vpnor/backend.cpp
index b155cf3..606acee 100644
--- a/vpnor/backend.cpp
+++ b/vpnor/backend.cpp
@@ -519,7 +519,7 @@
 
 struct backend backend_get_vpnor(void)
 {
-    struct backend be = {0};
+    struct backend be = {nullptr, nullptr, 0, 0, 0};
 
     be.ops = &vpnor_ops;