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/mboxd.c b/mboxd.c
index 059261b..04b2cb6 100644
--- a/mboxd.c
+++ b/mboxd.c
@@ -255,7 +255,8 @@
 			case '\0':
 				break;
 			case 'M':
-				context->backend.flash_size <<= 10;
+				context->backend.flash_size <<= 20;
+				break;
 			case 'K':
 				context->backend.flash_size <<= 10;
 				break;