[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.

scripts/bonnibel/source.py

@@ -3,6 +3,7 @@ class Action:
     implicit = property(lambda self: False)
     rule = property(lambda self: None)
     deps = property(lambda self: tuple())
+    parse_deps = property(lambda self: False)
 
     def __init__(self, name):
         self.__name = name
@@ -14,6 +15,7 @@ class Action:
 class Compile(Action):
     rule = property(lambda self: f'compile_{self.name}')
     deps = property(lambda self: ("${module_dir}/.parse_dep.phony",))
+    parse_deps = property(lambda self: True)
 
     def __init__(self, name, suffix = ".o"):
         super().__init__(name)