[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.

src/kernel/syscalls/endpoint.cpp

@@ -29,7 +29,9 @@ endpoint_send(j6_handle_t handle, uint64_t tag, const void * data, size_t data_l
 j6_status_t
 endpoint_receive(j6_handle_t handle, uint64_t * tag, void * data, size_t * data_len, uint64_t timeout)
 {
-    if (!tag || !data_len || (*data_len && !data))
+    // Data is marked optional, but we need the length, and if length > 0,
+    // data is not optional.
+    if (!data_len || (*data_len && !data))
         return j6_err_invalid_arg;
 
     endpoint *e = get_handle<endpoint>(handle);
@@ -46,7 +48,7 @@ endpoint_receive(j6_handle_t handle, uint64_t * tag, void * data, size_t * data_
 j6_status_t
 endpoint_sendrecv(j6_handle_t handle, uint64_t * tag, void * data, size_t * data_len, uint64_t timeout)
 {
-    if (!tag || (*tag & j6_tag_system_flag))
+    if (*tag & j6_tag_system_flag)
         return j6_err_invalid_arg;
 
     endpoint *e = get_handle<endpoint>(handle);