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/test/implicit_flush.c b/test/implicit_flush.c
index f186d19..41f27c5 100644
--- a/test/implicit_flush.c
+++ b/test/implicit_flush.c
@@ -148,4 +148,4 @@
 	flush_on_create(ctx);
 
 	return 0;
-};
+}