[kernel] Add automatic verification to syscalls

Since we have a DSL for specifying syscalls, we can create a verificaton
method for each syscall that can cover most argument (and eventually
capability) verification instead of doing it piecemeal in each syscall
implementation, which can be more error-prone.

Now a new _syscall_verify_* function exists for every syscall, which
calls the real implementation. The syscall table for the syscall handler
now maps to these verify functions.

Other changes:

- Updated the definition grammar to allow options to have a "key:value"
  style, to eventually support capabilities.
- Added an "optional" option for parameters that says a syscall will
  accept a null value.
- Some bonnibel fixes, as definition file changes weren't always
  properly causing updates in the build dep graph.
- The syscall implementation function signatures are no longer exposed
  in syscall.h. Also, the unused syscall enum has been removed.

assets/grammars/definitions.g

@@ -26,13 +26,14 @@ object_name: "object" name
 
 id: NUMBER
 name: IDENTIFIER
-options: "[" IDENTIFIER+ "]"
+options: "[" ( OPTION | IDENTIFIER )+ "]"
 description: COMMENT+
 
 PRIMITIVE: INT_TYPE | "size" | "string" | "buffer" | "address"
 INT_TYPE: /u?int(8|16|32|64)?/
 NUMBER: /(0x)?[0-9a-fA-F]+/
 UID: /[0-9a-fA-F]{16}/
+OPTION.2: IDENTIFIER ":" IDENTIFIER
 COMMENT: /#.*/
 PATH: /"[^"]*"/