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.h b/vpnor/backend.h
index 0102238..949e5c1 100644
--- a/vpnor/backend.h
+++ b/vpnor/backend.h
@@ -3,6 +3,7 @@
 #pragma once
 
 #include <limits.h>
+#include <string.h>
 
 struct mbox_context;
 struct vpnor_partition_table;