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/protocol.c b/protocol.c
index ab1c332..4efa83a 100644
--- a/protocol.c
+++ b/protocol.c
@@ -14,6 +14,9 @@
 #include "protocol.h"
 #include "windows.h"
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpointer-arith"
+#pragma GCC diagnostic ignored "-Wunused-result"
 
 #define BLOCK_SIZE_SHIFT_V1		12 /* 4K */
 
@@ -368,8 +371,9 @@
 
 static int generic_flush(struct mbox_context *context)
 {
-	int rc, i, offset, count;
+	int rc, offset, count;
 	uint8_t prev;
+	size_t i;
 
 	offset = 0;
 	count = 0;
@@ -617,8 +621,8 @@
 	return 0;
 }
 
-static int protocol_v2_flush(struct mbox_context *context,
-			     struct protocol_flush *io)
+static int protocol_v2_flush(struct mbox_context *context __attribute__((unused)),
+			     struct protocol_flush *io __attribute__((unused)))
 {
 	if (!(context->current && context->current_is_write)) {
 		MSG_ERR("Tried to call flush without open write window\n");
@@ -705,7 +709,7 @@
 	return 0;
 }
 
-void protocol_free(struct mbox_context *context)
+void protocol_free(struct mbox_context *context __attribute__((unused)))
 {
 	return;
 }
@@ -759,3 +763,5 @@
 
 	return 0;
 }
+
+#pragma GCC diagnostic pop